EXHIBIT 5.1
 
                           [ROPES & GRAY LETTERHEAD]
 
                               December 16, 1998
 
Eastern Enterprises
9 Riverside Road
Weston, MA 02493
 
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 Reorganization dated
as of October 17, 1998 (the "Merger Agreement"), by and between the Company and
Colonial 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