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                                                                   EX-99.906CERT

                            MFS/SUN LIFE SERIES TRUST

         CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT

     I, Richard M. Hisey, certify that, to my knowledge:

     1.   The Form N-CSR (the "Report") of MFS/Sun Life Series Trust (the
          "Registrant") fully complies for the period covered by the Report with
          the requirements of Section 13(a) or 15(d) of the Securities Exchange
          Act of 1934; and

     2.   The information contained in the Report fairly presents, in all
          material respects, the financial condition and results of operations
          of the Registrant.


Date: February 24, 2005               RICHARD M. HISEY
      -----------------               ------------------------------------------
                                      Richard M. Hisey
                                      Treasurer (Principal Financial Officer and
                                      Accounting Officer)


A SIGNED ORIGINAL OF THIS WRITTEN STATEMENT REQUIRED BY SECTION 906 HAS BEEN
PROVIDED TO THE REGISTRANT AND WILL BE RETAINED BY THE REGISTRANT AND FURNISHED
TO THE SECURITIES AND EXCHANGE COMMISSION OR ITS STAFF UPON REQUEST.

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                            MFS/SUN LIFE SERIES TRUST

         CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT

     I, Robert J. Manning, certify that, to my knowledge:

     1.   The Form N-CSR (the "Report") of MFS/Sun Life Series Trust (the
          "Registrant") fully complies for the period covered by the Report with
          the requirements of Section 13(a) or 15(d) of the Securities Exchange
          Act of 1934; and

     2.   The information contained in the Report fairly presents, in all
          material respects, the financial condition and results of operations
          of the Registrant.


Date: February 24, 2005               ROBERT J. MANNING
      -----------------               ------------------------------------------
                                      Robert J. Manning
                                      President (Principal Executive Officer)


A SIGNED ORIGINAL OF THIS WRITTEN STATEMENT REQUIRED BY SECTION 906 HAS BEEN
PROVIDED TO THE REGISTRANT AND WILL BE RETAINED BY THE REGISTRANT AND FURNISHED
TO THE SECURITIES AND EXCHANGE COMMISSION OR ITS STAFF UPON REQUEST.