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                                                                     EXHIBIT 5.1
 
                           [ROPES & GRAY LETTERHEAD]
 
                                  May 8, 1998
 
Eastern Enterprises
9 Riverside Road
Weston, MA 02193
 
     Re:  Eastern Enterprises
 
Ladies and Gentlemen:
 
     This opinion is furnished to you in connection with a registration
statement on Form S-4 (the "Registration Statement"), filed with the Securities
and Exchange Commission under the Securities Act of 1933, as amended, by Eastern
Enterprises, a Massachusetts business trust (the "Company"), for registration of
certain shares of Common Stock, $1.00 par value (the "Shares"), of the Company
to be issued pursuant to an Agreement and Plan of Merger, dated as of December
19, 1997 (the "Merger Agreement"), by and between the Company and Essex County
Gas Company, a Massachusetts gas utility company.
 
     We have acted as counsel for the Company in connection with the Merger
Agreement and the proposed issuance of the Shares thereunder. For purposes of
this opinion, we have examined and relied upon such documents, records,
certificates and other instruments as we have deemed necessary.
 
     We express no opinion as to the applicability of, compliance with or effect
of Federal law or the law of any jurisdiction other than The Commonwealth of
Massachusetts.
 
     Based on the foregoing, we are of the opinion that the Shares have been
duly authorized and, when the Shares have been issued and delivered in
accordance with the terms of the Merger Agreement, the Shares will be validly
issued, fully paid and non-assessable.
 
     We hereby consent to your filing this opinion as an exhibit to the
Registration Statement.
 
     It is understood that this opinion is to be used only in connection with
the Registration Statement while it is in effect and may not be relied upon for
any other purpose.
 
                                          Very truly yours,
 
                                          /s/ ROPES & GRAY
 
                                          Ropes & Gray