EXHIBIT 8.1
 
                           [ROPES & GRAY LETTERHEAD]
 
                               December 16, 1998
 
Eastern Enterprises
9 Riverside Road
Weston, MA 02493
 
Ladies and Gentlemen:
 
  We have acted as counsel to Eastern Enterprises, a Massachusetts business
trust ("Eastern"), in connection with the proposed merger (the "Merger") of
Colonial Gas Company, a Massachusetts corporation ("Colonial"), with and into a
wholly owned subsidiary of Eastern ("Merger Sub") pursuant to an Agreement and
Plan of Reorganization (the "Merger Agreement") dated as of October 17, 1998 by
and between Eastern and Colonial. The Merger is described in the Registration
Statement on Form S-4 (the "Registration Statement") of which this exhibit is a
part. This opinion is being rendered pursuant to the requirements of Item 21(a)
of Form S-4 under the Securities Act of 1933, as amended. All capitalized terms
used but not defined herein have the meanings ascribed to them in the Merger
Agreement.
 
  For purposes of the opinion set forth below, we have reviewed and relied upon
(i) the Merger Agreement, (ii) the Joint Proxy Statement and Prospectus (the
"Proxy Statement") included in the Registration Statement, (iii) the tax
representation letters delivered by Eastern and Colonial to us in connection
with this opinion, and (iv) such other documents, records and instruments as we
have deemed necessary or appropriate as a basis for our opinion. We have
assumed without investigation or verification that all statements contained in
the foregoing documents are true, correct and complete as of the date hereof
and will remain true, correct and complete through the Effective Time; that no
actions inconsistent with such statements have occurred or will occur; that all
such statements made "to the best of the knowledge of" any persons or parties,
or similarly qualified, are true, correct and complete as if made without such
qualification; and, as to all matters in which a person or entity making a
representation has represented that such person or entity either is not a party
to, does not have, or is not aware of, any plan or intention, understanding or
agreement, we have assumed that there is in fact no such plan, intention,
understanding or agreement.
 
  We also have assumed that (i) the Merger will be consummated in accordance
with the Merger Agreement (including satisfaction of all covenants and
conditions to the obligations of the parties without amendment or waiver
thereof); (ii) all representations and warranties contained in the Merger
Agreement are true, correct and complete in all respects; (iii) the Merger will
be effective as a merger under the applicable laws of Massachusetts; and (iv)
each of Eastern, Colonial and Merger Sub will comply with all reporting
obligations with respect to the Merger required under the Internal Revenue Code
(the "Code") and the Treasury regulations promulgated thereunder.
 
  Any inaccuracy in, or breach of, any of the aforementioned statements,
representations and assumptions and any change in applicable law after the date
hereof could adversely affect our opinion. No ruling has been sought from the
Internal Revenue Service by Eastern or Colonial as to the federal income tax
consequences of any aspect of the Merger, and the Internal Revenue Service is
not bound by our opinion herein.
 
  Based upon and subject to the foregoing, the discussion contained in the
Proxy Statement under the heading "THE MERGER--Certain Federal Income Tax
Consequences," subject to the limitations and qualifications described therein,
fairly and accurately represents our opinion as to the material federal income
tax consequences of the Merger.

 
  No opinion is expressed as to any matter not specifically addressed above,
including the tax consequences of any of the transactions under any foreign,
state, or local tax law or the tax consequences of any other transactions
contemplated or entered into by Eastern, Colonial or Merger Sub in connection
with the transactions described above. Our opinion is based on current federal
income tax law and we do not undertake to advise you as to any changes in
federal income tax law after the date hereof that may affect our opinion.
 
  This opinion is solely for your benefit, shall not inure to the benefit of
any other person, including without limitation any successor or assign of
Eastern, whether by operation of law or otherwise, and is not to be used,
circulated, quoted or otherwise referred to for any purpose without our express
written permission.
 
  We hereby consent to the use of our name under the caption "Federal Income
Tax Consequences" in the Registration Statement and to the filing with the
Securities and Exchange Commission of this opinion as an exhibit to the
Registration Statement on Form S-4 filed by Eastern in connection with the
Merger.
 
                                          Very truly yours,
                                          /s/ Ropes & Gray
                                          Ropes & Gray