MORGAN STANLEY MORTGAGE SECURITIES TRUST

c/o Morgan Stanley Investment Management Inc.
522 Fifth Avenue
New York, NY 10036

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

To Our Shareholders:

Notice is hereby given that a Special Meeting of Shareholders of Morgan Stanley Mortgage Securities Trust (the "Fund") will be held on Tuesday, November 13, 2012 at 9:00 a.m., at the offices of Morgan Stanley Investment Management Inc., 522 Fifth Avenue, 3rd Floor, Conference Room 3R, New York, NY 10036.

The Meeting is being held for the following purpose:

1.  To amend the Fund's fundamental investment restriction regarding concentration.

2.  To consider and act upon any other business as may properly come before the Meeting or any adjournment or postponement thereof.

Only holders of record of shares of the Fund as of the close of business on September 17, 2012, the record date for the Meeting, are entitled to notice of, and to vote at, the Meeting of the Fund or any adjournments or postponements thereof.

  MARY E. MULLIN
  Secretary

Dated: September [ ], 2012

You can help avoid the necessity and expense of sending follow-up letters to ensure a quorum by promptly returning the enclosed Proxy Card. If you are unable to be present in person, please fill in, sign and return the enclosed Proxy Card in order that the necessary quorum may be represented at the Meeting. The enclosed envelope requires no postage if mailed in the United States. Certain stockholders will be able to vote telephonically by touchtone telephone or electronically on the Internet by following instructions contained on their Proxy Card or on the enclosed Voting Information Card.

IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS FOR THE SPECIAL MEETING OF SHAREHOLDERS TO BE HELD ON NOVEMBER 13, 2012:

The Proxy Statement for the Special Meeting of Shareholders is available on the Internet at the website address located on the enclosed Proxy Card.

MORGAN STANLEY MORTGAGE SECURITIES TRUST

c/o Morgan Stanley Investment Management Inc.
522 Fifth Avenue
New York, NY 10036

PROXY STATEMENT

This Proxy Statement is furnished by the Board of Trustees (the "Board") of Morgan Stanley Mortgage Securities Trust (the "Fund") in connection with the solicitation of Proxies by the Board of Trustees for use at the Special Meeting of Shareholders of the Fund (the "Meeting") to be held on Tuesday, November 13, 2012, at 522 Fifth Avenue, 3rd Floor, Conference Room 3R, New York, NY 10036. It is expected that the Notice of Special Meeting, Proxy Statement and Proxy Card will first be mailed to shareholders of the Fund ("Shareholders") on or about September [ ], 2012. The purpose of the Meeting, the matters to be acted upon and the commencement time of the Meeting are set forth in the accompanying Notice of Special Meeting of Shareholders.

If the enclosed form of Proxy for the Fund is properly executed and returned in time, or is submitted by telephone or Internet, to be voted at the Meeting, the proxies named therein will vote the shares of beneficial interest with respect to the Fund (collectively, the "Shares") represented by it in accordance with the instructions marked thereon. Unmarked proxies submitted by Shareholders will be voted FOR Proposal 1. A Proxy may be revoked at any time prior to its exercise by any of the following: written notice of revocation to the Secretary of the Fund, execution and delivery of a later dated Proxy to the Secretary of the Fund (whether by mail or, as discussed below, by touchtone telephone or the Internet) (if returned and received in time to be voted), or attendance and voting at the Meeting. Attendance at the Meeting will not in and of itself revoke a Proxy. In order to revoke a Proxy in person, Shareholders must submit a subsequent Proxy. Shareholders whose Shares are held in street name by a broker of record and who wish to vote in person at the Meeting must obtain a legal proxy from their broker and present it at the Meeting to the inspector of elections.

The Board has fixed the close of business on September 17, 2012 as the record date for the determination of Shareholders entitled to notice of, and to vote at, the Meeting and at any adjournments or postponements thereof. On that date, the Fund had [ ] Shares outstanding and entitled to vote.

The cost of soliciting proxies for the Meeting, consisting principally of printing and mailing expenses, will be borne by the Fund. The solicitation of proxies will be by mail, which may be supplemented by solicitation by mail, telephone or otherwise through Trustees and officers of the Fund or officers and regular employees of Morgan Stanley Investment Management Inc. (the "Adviser"), the investment adviser for the Fund, Morgan Stanley & Co. LLC ("Morgan Stanley & Co.") and/or Morgan Stanley Smith Barney LLC, without special compensation therefor. In addition, the Fund may employ Computershare Fund Services ("Computershare") to make telephone calls to Shareholders to remind them to vote. The Fund may also employ Computershare as proxy solicitor if it appears that the required number of votes to achieve a quorum will not be received. The transfer agent services for the Fund are currently provided by Morgan Stanley Services Company Inc. (the "Transfer Agent").

Shareholders may be able to vote their Shares by touchtone telephone or by Internet by following the instructions on the Proxy Card accompanying this Proxy Statement. The Internet procedures are designed to authenticate a Shareholder's identity to allow Shareholders to vote their Shares and confirm that their instructions have been properly recorded. To vote by Internet or by touchtone telephone, Shareholders can access the website or call the toll-free number listed on the Proxy Card. To vote by touchtone telephone or by Internet, Shareholders will need the number that appears on the Proxy Card or Voting Information Card in the shaded box.

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In certain instances, Computershare may call Shareholders to ask if they would be willing to have their votes recorded by telephone. The telephone voting procedure is designed to authenticate Shareholders' identities, to allow Shareholders to authorize the voting of their Shares in accordance with their instructions and to confirm that their instructions have been recorded properly. No recommendation will be made as to how a Shareholder should vote on any Proposal other than to refer to the recommendations of the Board. The Fund has been advised by counsel that these procedures are consistent with the requirements of applicable law. Shareholders voting by telephone in this manner will be asked for identifying information and will be given an opportunity to authorize proxies to vote their Shares in accordance with their instructions. To ensure that the Shareholders' instructions have been recorded correctly, they will receive a confirmation of their instructions in the mail. A special toll-free number set forth in the confirmation will be available in case the information contained in the confirmation is incorrect. Although a Shareholder's vote may be taken by telephone, each Shareholder will receive a copy of this Proxy Statement and may vote by mail using the enclosed Proxy Card or by Internet or touchtone telephone as set forth above. The last proxy vote received in time to be voted, whether by Internet, mailed Proxy Card or touchtone telephone, will be the vote that is counted and will revoke all previous votes by the Shareholder. In the event that Computershare is retained as proxy solicitor, Computershare will be paid a project management fee as well as telephone solicitation expenses incurred for reminder calls, outbound telephone voting, confirmation of telephone votes, inbound telephone contact, obtaining Shareholders' telephone numbers and providing additional materials upon Shareholder request, at an estimated cost of $22,135, which would be borne by the Fund.

The Fund will furnish, without charge, a copy of its annual report for its fiscal year ended October 31, 2011 and its semi-annual report for the fiscal period ended April 30, 2012 to any Shareholder of the Fund requesting such report. Requests for annual and/or semi-annual reports should be made in writing to the Fund at Morgan Stanley Services Company Inc., P.O. Box 219886, Kansas City, MO 641219885, by calling toll-free (800) 869-6397 or by visiting the Adviser's Internet website at www.morganstanley.com/im.

Morgan Stanley Services Company Inc. serves as the Fund's administrator. The business address of Morgan Stanley Services Company Inc. is P.O. Box 219886, Kansas City, MO 641219885. State Street Bank and Trust Company serves as sub-administrator to the Fund. The business address of State Street Bank and Trust Company is One Lincoln Street, Boston, MA 02111-2101.

The Board of Trustees of the Fund recommend that you vote ''FOR'' Proposal No. 1.

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TO AMEND THE FUND'S FUNDAMENTAL INVESTMENT RESTRICTION REGARDING
CONCENTRATION
(Proposal No. 1)

Introduction

The Board of Trustees has approved a change to the Fund's policy regarding concentration that would permit the Fund to invest in excess of 25% of its assets in privately issued mortgage-backed securities. The current policy and the proposed policy are set forth below (proposed additional language is underlined).

Current Concentration Policy: The Fund will not invest more than 25% of the value of its total assets in securities of issuers in any one industry. This restriction does not apply to obligations issued or guaranteed by the U.S. government, its agencies or instrumentalities (the "Current Policy").

Proposed Concentration Policy: The Fund will not invest more than 25% of the value of its total assets in securities of issuers in any one industry. This restriction does not apply to obligations issued or guaranteed by the U.S. government, its agencies or instrumentalities or privately issued mortgage-backed securities (the "New Fundamental Policy").

Discussion of Proposal

The Investment Company Act of 1940, as amended (the "Investment Company Act"), requires a registered investment company, such as the Fund, to have "fundamental" policies governing certain of its investment practices. An investment policy is "fundamental" if it cannot be changed or deviated from without the approval of a "majority of the outstanding voting securities" (as defined in the Investment Company Act) of the fund. Investment companies may also voluntarily designate restrictions relating to other investment practices as fundamental. One such required fundamental policy relates to a fund's concentration of investments in a particular industry or group of related industries. The Investment Company Act does not define what constitutes "concentration" in an industry. However, the Securities and Exchange Commission (the "SEC") has taken the position that investment of 25% or more of a fund's total assets in one or more issuers conducting their principal business activities in the same industry (excluding the U.S. Government, its agencies or instrumentalities) constitutes concentration.

For purposes of applying the terms of the policy in the first sentence of both the Current Policy and the New Fundamental Policy (the "industry concentration policy"), mortgage-related or other asset-backed securities that are issued or guaranteed as to principal or interest by the U.S. Government or its agencies or instrumentalities are not subject to such policy, by virtue of the exclusion from that test available to all U.S. Government securities (as set forth in the second sentence of each Policy). If approved by Shareholders, for purposes of the industry concentration policy, privately issued mortgage-backed securities will be deemed not to represent ownership of securities in any particular "industry" or group of industries and therefore, the New Fundamental Policy will be interpreted to permit investment in such securities without limit.

To pursue its investment objective of seeking a high level of current income, the Fund normally invests at least 80% of its assets in mortgage-related securities. The Current Policy effectively limits the Fund's ability to invest in privately issued mortgage-backed securities to 25% of the Fund's assets. The Adviser believes that it is in the best interests of the Fund and its Shareholders to permit the Fund to invest in privately issued mortgage-backed securities in excess of 25% of its assets. In particular, the Adviser is of the view that increased flexibility to invest in privately issued mortgage-backed securities would enhance the Fund's ability to take advantage of current investment opportunities and facilitate its ability to respond to changes in market conditions. The Fund currently intends to invest up to 50% of its assets in privately issued mortgage-backed securities. Accordingly, the Adviser recommended to the Board that the New Fundamental Policy be adopted to replace the Current Policy. At its meeting on August 30, 2012, the Board considered the Adviser's recommendation. Upon evaluation of all facts

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it considered relevant, the Board unanimously approved the New Fundamental Policy and the submission of this Proposal to Shareholders for their approval.

If the Fund's Shareholders do not approve the New Fundamental Policy, the Fund's Current Policy will remain in effect.

Risks

The following describes risks associated with investing in privately issued mortgage-backed securities. The Fund already is subject to these risks because of its current investments in privately issued mortgage-backed securities. The Fund's prospectus and/or Statement of Additional Information will be revised to enhance the risk disclosure applicable to investments in privately issued mortgage-backed securities.

Pools created by non-governmental issuers generally offer a higher rate of interest than government and government-related pools because there are no direct or indirect government or agency guarantees of payments in such pools. Privately issued mortgage-backed securities are not subject to the same underwriting requirements for the underlying mortgages that are applicable to those mortgage-backed securities that have a Government or Government-sponsored entity guarantee. As a result, the mortgage loans underlying privately issued mortgage-backed securities may, and frequently do, have less favorable collateral, credit risk or other underwriting characteristics than Government or Government-sponsored mortgage-backed securities and have wider variances in a number of terms including interest rate, term, size, purpose and borrower characteristics. The risk of non-payment is greater for mortgage-backed securities that are backed by loans that were originated under weak underwriting standards, including loans made to borrowers with limited means to make repayment. A level of risk exists for all loans, although, historically, the poorest performing loans have been those classified as subprime. Privately issued mortgage-backed securities are not traded on an exchange and there may be a limited market for the securities, especially when there is a perceived weakness in the mortgage and real estate market sectors. Without an active trading market, mortgage-related securities held in the Fund's portfolio may be particularly difficult to value because of the complexities involved in assessing the value of the underlying mortgage loans. Privately issued mortgage-backed securities include securities that reflect an interest in, and are secured by, mortgage loans on commercial real property. Many of the risks of investing in commercial mortgage-backed securities reflect the risks of investing in the real estate securing the underlying mortgage loans. These risks reflect the effects of local and other economic conditions on real estate markets, the ability of tenants to make loan payments, and the ability of a property to attract and retain tenants.

The Fund may invest in privately issued mortgage-backed securities rated below investment grade which are commonly known as "junk" bonds" or "high yield securities." The Fund's investments in high yield securities pose significant risks. The prices of high yield securities are likely to be more sensitive to adverse economic changes or individual corporate developments than higher rated securities. During an economic downturn or substantial period of rising interest rates, junk bond issuers and, in particular, highly leveraged issuers may experience financial stress that would adversely affect their ability to service their principal and interest payment obligations, to meet their projected business goals or to obtain additional financing. In the event of a default, the Fund may incur additional expenses to seek recovery.

Vote Required

Approval of this Proposal requires the affirmative vote of the holders of a majority of the outstanding Shares of the Fund entitled to vote thereon. Under the Investment Company Act, this means the affirmative vote of the lesser of (a) 67% or more of the voting Shares present at the meeting or represented by proxy if the holders of more than 50% of the outstanding voting Shares are present or represented by proxy or (b) more than 50% of the outstanding voting Shares of the Fund.

The Board of Trustees of the Fund recommends that you vote ''FOR'' Proposal 1.

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SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS

[As of September 17, 2012, the following persons were known to own of record or beneficially 5% or more of the outstanding shares of a class of the Fund.]

[As of September 17, 2012, the aggregate number of Shares of the Fund owned by the Fund's officers and Trustees as a group was less than one percent of the Fund's outstanding Shares.]

ADDITIONAL INFORMATION

General

The holders of a majority of the Shares issued and outstanding and entitled to vote of the Fund, present in person or represented by proxy, will constitute a quorum at the Meeting. In the absence of a quorum, the Shareholders present or represented by proxy and entitled to vote thereat shall have the power to adjourn the Meeting from time to time. Where an adjournment is proposed because the necessary quorum to transact business is not obtained at the Meeting, the persons named as Proxies will vote in favor of such adjournment provided that such persons named as Proxies determine that such adjournment and additional solicitation is reasonable and in the interests of Shareholders based on all relevant factors, including the nature of the proposal, the percentage of Shareholders present, the nature of the proposed solicitation activities and the nature of the reasons for the further solicitation. The Shareholders present in person or represented by proxy at any meeting and entitled to vote thereat also shall have the power to adjourn the Meeting from time to time if the vote required to approve or reject the Proposal is not obtained (with Proxies being voted for or against adjournment consistent with the votes for and against the Proposal). The affirmative vote of the holders of a majority of the Shares then present in person or represented by proxy shall be required to adjourn the Meeting. Abstentions will not be voted either for or against any such adjournment.

Principal Underwriter and Administrator

Morgan Stanley Distribution, Inc., whose principal address is 522 Fifth Avenue, New York, NY 10036,, is the principal underwriter for the Fund. Morgan Stanley Services Company Inc., whose principal address is P.O. Box 219886, Kansas City, MO 64121-9886, serves as the Fund's administrator.

Submission of Shareholder Proposals

The Fund is not required and does not intend to hold regular shareholder meetings unless shareholder action is required in accordance with the Investment Company Act. Shareholders who would like to submit proposals for

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consideration at future shareholder meetings of the Fund should send written proposals to Mary E. Mullin, Secretary, 522 Fifth Avenue, New York, NY 10036. To be considered for presentation at a shareholders' meeting, rules promulgated by the SEC require that, among other things, a shareholder's proposal must be received at the offices of the Fund within a reasonable time before a solicitation is made. Timely submission of a proposal does not necessarily mean that such proposal will be included.

OTHER MATTERS

The Board knows of no business, other than that set forth in the Notice of Special Meeting of Shareholders, to be presented for consideration at the Meeting. However, the Proxy confers discretionary authority upon the persons named therein to vote as they determine on other business, not currently contemplated, which may come before the Meeting.

  MARY E. MULLIN
  Secretary

Dated: September [ ], 2012

Shareholders of the Fund who do not expect to be present at the Meeting for the Fund and who wish to have their Shares voted are requested to vote their Shares over the Internet, by telephone or by dating and signing the enclosed Proxy Card for the Fund and returning it in the enclosed envelope. No postage is required if mailed in the United States.

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