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                                                                   Exhibit 5.1


                     [LETTERHEAD OF FOLEY, HOAG & ELIOT LLP]






                                February 25, 1998


Telco Systems, Inc.
63 Nahatan Street
Norwood, Massachusetts  02062

Ladies and Gentlemen:

         We are familiar with the Registration Statement on Form S-3 (the "S-3
Registration Statement") filed today by Telco Systems, Inc., a Delaware
corporation (the "Company"), with the Securities and Exchange Commission under
the Securities Act of 1933, as amended. The S-3 Registration Statement relates
to the proposed offering by the Company of 101,636 shares (the "Shares") of its
Common Stock, par value $0.01 per share, all of which Shares are now issued and
outstanding.

         In arriving at the opinions expressed below, we have examined and
relied on the following documents: (a) the Certificate of Incorporation of the
Company; (b) the By-Laws of the Company, as amended; and (c) the records of
meetings and consents of the Board of Directors and stockholders of the Company
provided to us by the Company. In addition, we have examined and relied on the
originals or copies certified or otherwise identified to our satisfaction of all
such corporate records of the Company and such other instruments and other
certificates of public officials, officers and representatives of the Company
and such other persons, and we have made such investigations of law, as we have
deemed appropriate as a basis for the opinions expressed below.


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Telco Systems, Inc.
February 25, 1998
Page 2


         Based upon the foregoing, it is our opinion that:

         1.   The Company has taken all necessary corporate action required to
authorize the issuance and sale of the Shares; and

         2.   The Shares have been validly and legally issued and are fully paid
and non-assessable.

         We hereby consent to the filing of this opinion as an exhibit to the
S-3 Registration Statement.


                                                  Very truly yours,

                                                  FOLEY, HOAG & ELIOT LLP



                                                  By: /s/ Edward N. Gadsby, Jr.
                                                      -------------------------
                                                      A Partner




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