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                                                                       EXHIBIT 5

                               PALMER & DODGE LLP
                                One Beacon Street
                           Boston, Massachusetts 02108

Telephone: (617) 573-0100                              Facsimile: (617) 227-4420


                                  June 29, 2000


GelTex Pharmaceuticals, Inc.
153 Second Avenue
Waltham, MA 02451

Ladies and Gentlemen:

     We are rendering this opinion in connection with the Registration Statement
on Form S-8 (the "Registration Statement") filed by GelTex Pharmaceuticals, Inc.
(the "Company") with the Securities and Exchange Commission (the "Commission")
under the Securities Act of 1933, as amended (the "Act"), on or about the date
hereof. The Registration Statement relates to 50,000 shares (the "Plan Shares")
of the Company's Common Stock, $0.01 par value, offered pursuant to the
provisions of the Company's 1995 Employee Stock Purchase Plan (the "Plan").

     We have acted as your counsel in connection with the preparation of the
Registration Statement and are familiar with the proceedings taken by the
Company in connection with the authorization of the issuance and sale of the
Plan Shares. We have examined all such documents as we consider necessary to
enable us to render this opinion.

     Based upon the foregoing, we are of the opinion that when issued in
accordance with the terms of the Plan, the Plan Shares will be duly authorized,
validly issued, fully paid, and nonassessable.

     We hereby consent to the filing of this opinion as a part of the
Registration Statement.

                                                     Very truly yours,


                                                     /s/ Palmer & Dodge LLP
                                                     PALMER & DODGE LLP