As filed with the Securities and Exchange Commission on June 26, 2007
                                                   1933 Act File No. 333-136796

================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                  -------------

                                    FORM N-14

                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

[__] Pre-Effective Amendment No. _______ [ X ] Post-Effective Amendment No. 1

                             MFS(R) SERIES TRUST IX
               (Exact Name of Registrant as Specified in Charter)

                500 Boylston, Street, Boston, Massachusetts 02116
                    (Address of Principal Executive Offices)
        Registrant's Telephone Number, Including Area Code: 617-954-5000

                                  -------------

                              Susan S. Newton, Esq.
                    Massachusetts Financial Services Company,
                               500 Boylston Street
                           Boston, Massachusetts 02116
                     (Name and Address of Agent for Service)

                                  -------------

                  APPROXIMATE DATE OF PROPOSED PUBLIC OFFERING:
              IT IS PROPOSED THAT THIS FILING WILL BECOME EFFECTIVE
                IMMEDIATELY UPON FILING PURSUANT TO RULE 485(b).

Title of Securities Being Registered: Class A, Class B, Class C and Class I
shares of beneficial interest in the series of the Registrant designated MFS
Limited Maturity Fund, a series of MFS Series Trust IX.

NO FILING FEE IS REQUIRED BECAUSE AN INDEFINITE NUMBER OF SHARES HAVE PREVIOUSLY
BEEN REGISTERED PURSUANT TO RULE 24F-2 UNDER THE INVESTMENT COMPANY ACT OF 1940.


===============================================================================




                          MFS(R) LIMITED MATURITY FUND

                       CONTENTS OF REGISTRATION STATEMENT

                           This Registration Statement contains the following
papers and documents:

o        Cover Sheet
o        Contents of Registration Statements
o        Part A - Prospectus/Proxy Statement*
o        Part B - Statement of Additional Information*
o        Part C - Other Information
o        Signature Page
o        Exhibits

     * Previously filed in Registrant's Registration Statement on Form N-14, SEC
     File No. 333-136796 on August 21, 2006 and subsequently filed in definitive
     form pursuant to Rule 497.

     This post-effective amendment is being filed solely for the purpose of
     including in the Registration Statement the definitive Plan of
     Reorganization (Exhibit 4) and the Opinion and Consent of Counsel on Tax
     Matters (Exhibit 12) for the reorganization of MFS Government Limited
     Maturity Fund into MFS Limited Maturity Fund, a series of the Registrant.


                                    FORM N-14
                                     PART C
                                OTHER INFORMATION



Item 15. Indemnification

                  Reference is hereby made to (a) Article V of the Trust's
Amended and Restated Declaration of Trust, dated as of December 16, 2004,
incorporated by reference to Registrant's Post-Effective Amendment No. 56 filed
with the SEC via EDGAR on February 15, 2005 and (b) Section 8 of the Shareholder
Servicing Agent Agreement, incorporated by reference to Registrant's
Post-Effective Amendment No. 32 filed with the SEC via EDGAR on August 28, 1995.

                  The Trustees and officers of the Registrant and the personnel
of the Registrant's investment adviser and principal underwriter are insured
under an errors and omissions liability insurance policy. The Registrant and its
officers are also insured under the fidelity bond required by Rule 17g-1 under
the Investment Company Act of 1940, as amended.

                  Insofar as indemnification for liability arising under the
Securities Act of 1933 may be permitted to trustees, officers and controlling
persons of the Registrant


pursuant to the foregoing  provisions,  or otherwise,  the  Registrant  has been
advised  that in the opinion of the  Securities  and  Exchange  Commission  such
indemnification  is  against  public  policy  as  expressed  in such Act and is,
therefore,  unenforceable. In the event that a claim for indemnification against
such liabilities  (other than the payment by the Registrant of expenses incurred
or paid by a trustee,  officer or  controlling  person of the  Registrant in the
successful  defense of any  action,  suit or  proceeding)  is  asserted  by such
trustee,  officer or controlling  person in connection with the securities being
registered hereby, the Registrant will, unless in the opinion of its counsel the
matter  has  been  settled  by  controlling  precedent,  submit  to a  court  of
appropriate  jurisdiction  the question  whether such  indemnification  by it is
against public policy as expressed in such Act and will be governed by the final
adjudication of such issue.

Item 16.          EXHIBITS

                   1  (a)    Amended and Restated Declaration of Trust, dated
                             December 16, 2004.  (1)

                      b)     Amendment, dated February 1, 2005, to the
                             Declaration of Trust to terminate MFS Emerging
                             Opportunities Fund. (1)

                      (c)    Amendment, dated March 10, 2005, to the Declaration
                             of Trust - Establishment and Designation of Class R
                             Shares, Class R3 Shares, Class R4 Shares and Class
                             R5 Shares for MFS Bond Fund, MFS Inflation-Adjusted
                             Bond Fund, MFS Intermediate Investment Grade Bond
                             Fund, MFS Limited Maturity Fund and MFS Research
                             Bond Fund. (24)

                      (d)    Amendment, dated April 1, 2005, to the Declaration
                             of Trust - Redesignation of Class R1 Shares as
                             Class R Shares and of Class R2 Shares as Class R3
                             Shares for MFS Bond Fund, MFS Inflation-Adjusted
                             Bond Fund, MFS Intermediate Investment Grade Bond
                             Fund, MFS Limited Maturity Fund and MFS Research
                             Bond Fund. (24)

                      (e)    Amendment, dated February 22, 2006, to the
                             Declaration of Trust - Establishment and
                             Designation of Class W Shares for MFS Research Bond
                             Fund. (12)

                      (f)    Amendment, dated March 30, 2007, to the Declaration
                             of Trust - Termination of Class 529A, 529B, 529C
                             shares of MFS Bond Fund and MFS Inflation-Adjusted
                             Bond Fund, (22)

                   2  (a)    Amended and Restated By-Laws, dated January 1,
                             2002, as revised June 23, 2004.  (23)

                      (b)    Appendix A, dated June 28, 2005, as revised
                             February 27, 2007, to the Master Amended and
                             Restated By-Laws, dated January 1, 2002. (19)

                   3         Not Applicable.


                   4         Plan of Reorganization; included as Exhibit A to
                             MFS Government Limited Maturity Fund
                             Prospectus/Proxy Statement set forth in Part A to
                             the Registration Statement on Form N-14; filed
                             herewith.

                   5         Copies of instruments defining the rights of
                             shareholders, including the relevant portions of:
                             the Amended and Restated Declaration of Trust,
                             dated December 16, 2004, as amended through March
                             30, 2007 (see Section 6.2), and the Amended and
                             Restated By-Laws, dated January 1, 2002 as revised
                             June 23, 2004 (see Article III). (28)

                   6         Investment Advisory Agreement for the Trust, dated
                             January 1, 2002.  (18)

                   7  (a)    Distribution Agreement, dated January 1, 1995.  (4)

                      (b)    Dealer Agreement between MFS Fund Distributors,
                             Inc. ("MFD") and a dealer as of September, 2004;
                             the Mutual Fund Agreement between MFD and a trust
                             institutional effective May 2002; Mutual Fund
                             Agreement; Supplement to Mutual Fund Agreement;
                             Amended and Restated MFS Serviced Plan Supplement
                             to Dealer or Mutual Fund Agreement; Notice of
                             Amendment to Dealer or Mutual Fund Agreement
                             effective March, 2005, and Rule 22c-2 Supplement to
                             Dealer Agreement or Mutual Fund Agreement. (17)

                   8  (a)    Master Retirement Plan for Non-Interested Person
                             Trustees, dated January 1, 1991, as
                             amended and restated February 17, 1999.  (25)

                      (b)    Amendment to the Master Retirement Plan for
                             Non-Interested Person Trustees, dated July 1, 2002.
                             (11)

                      (c)    Retirement Benefit Deferral Plan, dated July 1,
                             2002. (11)

                   9  a)     Master Custodian Contract between the Registrant
                             and JP Morgan Chase Bank, N.A., dated
                             November 13, 2006.  (15)

                      (b)    Appendix A to the Master Custodian Agreement
                             between the Registrant and JP Morgan Chase Bank,
                             N.A., dated as of February 27, 2007. (19)

                      (c)    Fund Accounting Agreement between the Registrant
                             and JP Morgan Investor Services Co., dated November
                             13, 2006. (15)

                      (d)    Appendix A to the Fund Accounting Agreement between
                             the Registrant and JP Morgan Investor Services Co.,
                             dated as of February 27, 2007. (19)

                  10  (a)    Master Distribution Plan pursuant to Rule 12b-1
                             under the Investment Company Act of 1940
                             effective January 1, 1997, Amended and Restated
                             effective October 25, 2006.  (21)


                      (b)    Plan pursuant to Rule 18f-3(d) under the Investment
                             Company Act of 1940 effective September 6, 1996, as
                             amended and restated effective October 25, 2006.
                             (21)

                  11         Opinion of Susan Newton, including consent,
                             dated August 18, 2006.  (26)

                  12         Opinion of Ropes & Gray LLP as to tax matters,
                             including consent; filed herewith.

                  13  (a)    Shareholder Servicing Agent Agreement, dated
                             December 2, 1985.  (4)

                      (b)    Amendment to Exhibit B of the Shareholder Servicing
                             Agent Agreement to amend fee schedule dated April
                             1, 2003. (20)

                      (c)    Amendment to Shareholder Servicing Agreements,
                             dated February 22, 2005. (16)

                      (d)    Master Administrative Services Agreement, dated
                             March 1, 1997 as amended and restated August 1,
                             2006. (14)

                      (e)    Exhibit A, as revised February 27, 2007, to the
                             Amended and Restated Master Administrative Services
                             Agreement. (19)

                      (f)    Master 529 Administrative Services Agreement, dated
                             August 1, 2002.  (7)

                      (g)    Exhibit A, as revised February 27, 2007, to the
                             Master 529 Administrative Services Agreement, dated
                             August 1, 2002. (22)

                      (h)    Addendum, dated October 16, 2002, to the Master 529
                             Administrative Services Agreement. (7)

                      (i)    Master Class R Plan Administration and Service
                             Agreement, dated April 1, 2005. (6)

                      (j)    Exhibit A, dated July 26, 2005, to the Master Class
                             R Plan Administration and Service Agreement. (23)

                      (k)    Exhibit C, dated April 1, 2007 to the Master Class
                             R Plan Administration and Service Agreement. (3)

                      (l)    Special Servicing Agreement, dated May 1, 2007. (2)

                  14  (a)    Auditor's Consent Letter for Ernst & Young LLP
                             regarding MFS Government Limited Maturity
                             Fund. (26)


                      (b)    Auditor's Consent Letter for Deloitte & Touche LLP
                             regarding MFS Limited Maturity Fund. (26)

                  16  (a)    Power of Attorney, dated September 1, 2005.  (23)

                      (b) Power of Attorney, dated November 1, 2005. (9)

                      (c) Power of Attorney, dated April 25, 2006. (12)

                  17  (a)    MFS Government Limited Maturity Fund's Prospectus
                             and Statement of Additional Information,
                             dated May 1, 2006.  (13)

                      (b)    MFS Government Limited Maturity Fund's Annual
                             Report to Shareholders for the fiscal year ended
                             December 31, 2005. (10)

                      (c)    MFS Government Limited Maturity Fund's Semi-Annual
                             Report to Shareholders for the period ended June
                             30, 2006. (27)

                      (d)    MFS Limited Maturity Fund's Prospectus and
                             Statement of Additional Information, dated
                             September 1, 2006. (8)

                      (e)    MFS Limited Maturity Fund's Annual Report to
                             Shareholders for the fiscal year ended April 30,
                             2006. (5)
- -------------------------
(1)  Incorporated by reference to the Registrant's  Post-Effective Amendment No.
     56 filed with the SEC via EDGAR on February 15, 2005.
(2)  Incorporated by reference to MFS Series Trust XII (File Nos. 333-126328 and
     811-21780)  Post-Effective  Amendment No. 7 filed with the SEC via EDGAR on
     June 28, 2007.
(3)  Incorporated  by reference  to MFS Series Trust III (File Nos.  2-60491 and
     811-2794)  Post-Effective  Amendment No. 40 filed with the SEC via EDGAR on
     March 29, 2007.
(4)  Incorporated by reference to the Registrant's  Post-Effective Amendment No.
     32 filed with the SEC via EDGAR on August 28, 1995.
(5)  Incorporated  by reference  to MFS Series  Trust IX (File Nos.  2-50409 and
     811-2464) N-CSR filed with the SEC via EDGAR on July 7, 2006.
(6)  Incorporated  by reference  to MFS Series Trust III (File Nos.  2-60491 and
     811-2794)  Post-Effective  Amendment No. 37 filed with the SEC via EDGAR on
     March 31, 2005.
(7)  Incorporated  by  reference  to MFS Series  Trust X (File Nos.  33-1657 and
     811-4492)  Post-Effective  Amendment No. 43 filed with the SEC via EDGAR on
     November 27, 2002.
(8)  Incorporated  by reference  to MFS Series  Trust IX (File Nos.  2-50409 and
     811-2464)  Post-Effective  Amendment No. 63 filed with the SEC via EDGAR on
     August 25, 2006.
(9)  Incorporated  by  reference  to MFS Series  Trust X (File Nos.  33-1657 and
     811-4492)  Post-Effective  Amendment No. 58 filed with the SEC via EDGAR on
     November 28, 2005.
(10) Incorporated  by reference to MFS  Government  Limited  Maturity Fund (File
     Nos. 2-96738 and 811-4253) Form N-CSR filed with the SEC via EDGAR on March
     10, 2006.
(11) Incorporated  by reference  to MFS Series  Trust IX (File Nos.  2-50409 and
     811-2464)  Post-Effective  Amendment No. 44 filed with the SEC via EDGAR on
     August 1, 2002.
(12) Incorporated  by reference  to MFS Series  Trust IX (File Nos.  2-50409 and
     811-2464)  Post-Effective  Amendment No. 62 filed with the SEC via EDGAR on
     April 27, 2006.
(13) Incorporated  by reference to MFS  Government  Limited  Maturity Fund (File
     Nos. 2-96738 and 811-4253)  Post-Effective  Amendment No. 27 filed with the
     SEC via EDGAR on April 28, 2006.
(14) Incorporated  by  reference  to MFS Series  Trust X (File Nos.  33-1657 and
     811-4492)  Post-Effective  Amendment No. 62 filed with the SEC via EDGAR on
     September 29, 2006.
(15) Incorporated  by reference  to MFS Series  Trust II (File Nos.  33-7637 and
     811-4775)  Post-Effective  Amendment No. 30 filed with the SEC via EDGAR on
     January 26, 2007.


(16) Incorporated  by  reference  to MFS Series  Trust X (File Nos.  33-1657 and
     811-4492)  Post-Effective  Amendment No. 55 filed with the SEC via EDGAR on
     March 14, 2005.
(17) Incorporated by reference to MFS Series Trust VIII (File Nos.  33-37972 and
     811-5262)  Post-Effective  Amendment No. 28 filed with the SEC via EDGAR on
     December 28, 2006.
(18) Incorporated by reference to Registrant's  Post-Effective  Amendment No. 43
     filed with the SEC via EDGAR on July 30, 2002.
(19) Incorporated  by reference to MFS Series  Trust XIV (File.  No.  811-22033)
     Registration  Statement  on Form N-1A filed with the SEC via EDGAR on March
     15, 2007.
(20) Incorporated by reference to Registrant's  Post-Effective  Amendment No. 48
     filed with the SEC via EDGAR on June 27, 2003.
(21) Incorporated by reference to MFS Municipal  Series Trust (File Nos. 2-92915
     and 811-4096)  Post-Effective Amendment No. 46 filed with the SEC via EDGAR
     on March 20, 2007.
(22) Incorporated by reference to Registrant's  Post-Effective  Amendment No. 65
     filed with the SEC via EDGAR on June 28, 2007.
(23) Incorporated  by  reference  to MFS Series  Trust X (File Nos.  33-1657 and
     811-4492)  Post-Effective  Amendment No. 57 filed with the SEC via EDGAR on
     September 28, 2005.
(24) Incorporated by reference to Registrant's  Post-Effective  Amendment No. 58
     filed with the SEC via EDGAR on March 31, 2005.
(25) Incorporated  by  reference  to MFS  Growth  Opportunities  Fund (File Nos.
     2-36431 and  811-2032)  Post-Effective  Amendment No. 39 filed with the SEC
     via EDGAR on February 26, 1999.
(26) Previously  filed.  Incorporated  by reference from the exhibits filed with
     Registrant's  Initial  Registration   Statement  on  Form  N-14  (File  No.
     333-136796), as filed with the SEC via EDGAR on August 21, 2006.
(27) Incorporated  by reference to MFS  Government  Limited  Maturity Fund (File
     Nos.  2-96738  and  811-4253)  Form  N-CSR  filed with the SEC via EDGAR on
     August 31, 2006.
(28) Amended and Restated  Declaration  of Trust,  dated  December 16, 2004,  as
     amended   through   February  22,  2006,   incorporated   by  reference  to
     Post-Effective  Amendment  No. 56 to  Registrant's  Registration  Statement
     filed with the SEC via EDGAR on February  15,  2005;  Amended and  Restated
     By-Laws,  dated January 1, 2002, as revised June 23, 2004  incorporated  by
     reference  to  MFS  Series  Trust  X  (File  Nos.   33-1657  and  811-4492)
     Post-Effective  Amendment  No. 57 filed with the SEC via EDGAR on September
     28, 2005.

Item 17.          UNDERTAKINGS

                  (a) The undersigned Registrant agrees that prior to any public
reoffering of the securities registered through the use of a prospectus which is
a part of this Registration Statement by any person or party who is deemed to be
an underwriter within the meaning of Rule 145(c) under the Securities Act, the
reoffering prospectus will contain the information called for by the applicable
registration form for reofferings by persons who may be deemed underwriters, in
addition to the information called for by the other items of the applicable
form.

                  (b) The undersigned Registrant agrees that every prospectus
that is filed under paragraph (a) above will be filed as a part of an amendment
to this Registration Statement and will not be used until the amendment is
effective, and that, in determining any liability under the 1933 Act, each
post-effective amendment shall be deemed to be a new Registration Statement for
the securities offered therein, and the offering of the securities at that time
shall be deemed to be the initial bona fide offering of them.

(c) The Registration agrees to file an executed copy of an opinion of
counsel supporting the tax consequences of the proposed reorganization as an
amendment to this Registration Statement within a reasonable time after receipt
of such opinion.







                                     NOTICE


                  A copy of the Amended and Restated Declaration of Trust, as
amended, is on file with the Secretary of State of The Commonwealth of
Massachusetts, and notice is hereby given that this Registration Statement has
been executed on behalf of the Registrant by an officer of the Registrant as an
officer and not individually, and the obligations of or arising out of this
Registration Statement are not binding upon any of the Trustees, officers, or
shareholders of the Registrant individually, but are binding only upon the
assets and property of the Registrant.






                                   SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
has duly caused this Registration Statement to be signed on its behalf by the
undersigned, thereto duly authorized, in the City of Boston and The Commonwealth
of Massachusetts on the 26th day of June 2007.

                                      MFS(R) SERIES TRUST IX

                                      By:      MARIA F. DWYER*
                                      -----------------------------------------
                                      Name:    Maria F. Dwyer
                                      Title:   President

      Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement on Form N-14 has been signed below by the following
persons in the capacities indicated on June 26, 2007.

             SIGNATURE                          TITLE

MARIA F. DWYER*                       President (Principal Executive Officer)
- -------------------------
Maria F. Dwyer


TRACY A. ATKINSON*                    Principal Financial and Accounting Officer
- -------------------------
Tracy A. Atkinson


ROBERT E. BUTLER*                     Trustee
- -------------------------
Robert E. Butler


LAWRENCE H. COHN*                     Trustee
- -------------------------
Lawrence H. Cohn


DAVID H. GUNNING*                     Trustee
- -------------------------
David H. Gunning


WILLIAM R. GUTOW*                     Trustee
- -------------------------
William R. Gutow


MICHAEL HEGARTY*                      Trustee
- -------------------------
Michael Hegarty







J. ATWOOD IVES*                       Trustee
- -------------------------
J. Atwood Ives


ROBERT J. MANNING*                    Trustee
- -------------------------
Robert J. Manning


LAWRENCE T. PERERA*                   Trustee
- -------------------------
Lawrence T. Perera


ROBERT C. POZEN*                      Trustee
- -------------------------
Robert C. Pozen


J. DALE SHERRATT*                     Trustee
- -------------------------
J. Dale Sherratt


LAURIE J. THOMSEN*                    Trustee
- -------------------------
Laurie J. Thomsen


ROBERT W. UEK*                        Trustee
- -------------------------
Robert W. Uek

                                     *By:      SUSAN S. NEWTON
                                     ----------------------------------------
                                     Name:     Susan S. Newton
                                              as Attorney-in-fact

                                     Executed  by Susan S.  Newton on  behalf of
                                     those indicated  pursuant to (i) a Power of
                                     Attorney, dated September 1, 2005;
                                     incorporated  by  reference to MFS Series
                                     Trust X (File  Nos.  33-1657  and 811-4492)
                                     Post-Effective Amendment No. 57 filed with
                                     the SEC via EDGAR on September 28, 2005
                                     (Atkinson);  (ii) a Power of  Attorney,
                                     dated  November 1, 2005,  incorporated  by
                                     reference  to MFS  Series  Trust X (File
                                     Nos. 33-1657 and 811-4492)  Post-Effective
                                     Amendment  No. 58 filed  with the SEC via
                                     EDGAR on  November  28, 2005  (Dwyer);  and
                                     (iii)  a  Power  of   Attorney,   dated
                                     April 25,  2006;  incorporated by reference
                                     to MFS Series  Trust IX (File  Nos. 2-50409
                                     and 811-2464) Post-Effective Amendment No.
                                     61  filed  with  the  SEC  via  EDGAR  on
                                     April  27,   2006 (Trustees).






                                INDEX TO EXHIBITS


EXHIBIT NO.                              DESCRIPTION OF EXHIBIT

     4      Plan of Reorganization; included as Exhibit A to MFS Government
            Limited Maturity Fund Prospectus/Proxy Statement set forth in Part A
            to the Registration Statement on Form N-14.

    12      Opinion of Ropes & Gray LLP as to tax matters, including consent.