Exhibit 99.906Cert

FORM N-CSR ITEM 11(b) EXHIBIT

                            CERTIFICATION PURSUANT TO
                             18 U.S.C. SECTION 1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

          The  undersigned hereby certify in their capacity as Treasurer and
President, respectively, of TAX-MANAGED MID-CAP CORE PORTFOLIO (the "Portfolio")
that:

     (a)  the Annual Report of the Portfolio on Form N-CSR for the period ended
          October 31, 2004 (the "Report") fully complies with the requirements
          of Section 13(a) or 15(d) of the Securities Exchange Act of 1934, as
          amended; and

     (b)  the information contained in the Report fairly presents, in all
          material respects, the financial condition and the results of
          operations of the Portfolio for such period.

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


Tax-Managed Mid-Cap Core Portfolio
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Date: December 22, 2004
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/s/ Michelle A. Green
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Michelle A. Green
Treasurer

Date: December 22, 2004
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/s/ James B. Hawkes
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James B. Hawkes
President