EXHIBIT NO. 99.11

                                    M F S(SM)
                             INVESTMENT MANAGEMENT

                            MFS INVESTMENT MANAGEMENT
              500 Boylston Street, Boston, Massachusetts 02116-3741
                                  617-954-5000

Susan S. Newton
Senior Vice President and Associate General Counsel
Massachusetts Financial Services Company
500 Boylston Street
Boston, MA  02116
Phone: (617) 954-5182
Fax:   (617) 954-7760

December 18, 2006

MFS Municipal Income Fund
A Series of MFS Municipal Series Trust

Ladies and Gentlemen:

I have acted as counsel to the MFS Municipal Income Fund (the "Municipal Income
Fund"), a series of MFS Municipal Series Trust, a Massachusetts business trust
(the "Trust"), in connection with the Trust's Registration Statement on Form
N-14 to be filed with the Securities and Exchange Commission (the "Commission")
on or about December 18, 2006 (the "Registration Statement"), with respect to an
indefinite number of Shares of Beneficial Interest (no par value) (the "Shares")
of the Municipal Income Fund to be issued pursuant to an Agreement and Plan of
Reorganization (the "Agreement and Plan") adopted by MFS Series Trust IV, a
Massachusetts business trust, on behalf of MFS Municipal Bond Fund, a series of
MFS Series Trust IV, and the Trust on behalf of the Municipal Income Fund.

       In onnection with this opinion, I have examined the following documents:

       (a)  the Registration Statement;

       (b)  the Agreement and Plan;

       (c)  a certificate of the Secretary of State of The Commonwealth of
            Massachusetts as to the existence of the Trust;

       (d)  copies of the Trust's Amended and Restated Declaration of Trust
            ("Declaration of Trust") and of all amendments thereto on file in
            the office of the Secretary of State; and

       (e)  the Trust's Amended and Restated By-Laws and certain votes of the
            Trustees of the Trust.

       In such examination, I have assumed the genuineness of all signatures,
the conformity to the originals of all of the documents reviewed by me as
copies, the authenticity and completeness of all original documents reviewed by
me in original or copy form and the legal competence of each individual
executing any document. I have also assumed, for the purposes of this opinion,
that the Agreement and Plan, in substantially the form reviewed by me, is duly
delivered by the parties thereto and that all of the conditions set forth in
"Information About the Reorganization" in the Registration Statement shall have
occurred prior to the issuance and sale of the Shares.

       This opinion is based entirely on my review of the documents listed
above. I have made no other review or investigation of any kind whatsoever, and
I have assumed, without independent inquiry, the accuracy of the information set
forth in such documents.

       This opinion is limited solely to the laws of The Commonwealth of
Massachusetts (other than the Massachusetts Uniform Securities Act, as to which
I express no opinion) as applied by courts in such Commonwealth.

       I understand that all of the foregoing assumptions and limitations are
acceptable to you.

       Based upon and subject to the foregoing, please be advised that it is my
opinion that the Shares, when issued and sold in accordance with the
Registration Statement, the Agreement and Plan and the Trust's Declaration of
Trust and By-laws, will be legally issued, fully paid and non-assessable, except
that shareholders of the Trust may under certain circumstances be held
personally liable for the Trust's obligations.

       A copy of the Trust's Declaration of Trust is on file with the Secretary
of State of the Commonwealth of Massachusetts. I note specifically that the
obligations of or arising out of the Agreement and Plan are not binding upon any
of the Trust's trustees, officers, employees, agents or shareholders
individually, but are binding solely upon the assets and property of the Trust
in accordance with its interest under the Agreement and Plan. I further note
that the assets and liabilities of each series of the Trust, such as the
Municipal Income Fund, are separate and distinct and that the Municipal Income
Fund's obligations of or arising out of the Agreement and Plan are binding
solely upon the assets or property of the Municipal Income Fund

       I hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.

                        Very truly yours,

                        SUSAN S. NEWTON
                        ---------------
                        Susan S. Newton