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                                                                     Exhibit 5.1

                               Hale and Dorr LLP
                               Counsellors at Law
                  60 State Street, Boston, Massachusetts 02109
                        617-526-6000 . fax 617-526-5000

                                          June 1, 1999

Arch Communications Group, Inc.
1800 West Park Drive, Suite 250
Westborough, MA 01581

  Re: Registration Statement on Form S-1

Ladies and Gentlemen:

  This opinion is furnished to you in connection with a Registration Statement
on Form S-1 (the "Registration Statement") filed with the Securities and
Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended (the "Securities Act"), for the registration of (i) 152 shares of
Common Stock, $.01 par value per share (the "Common Stock"), of Arch
Communications Group, Inc., a Delaware corporation (the "Company") previously
held by Credit Suisse First Boston Corporation and (ii) up to 3,675,659 shares
of the Company's Common Stock issuable upon exercise of warrants to purchase
Common Stock of the Company (collectively, the "Securities").

  We are acting as counsel for the Company in connection with the issuance by
the Company of the Securities. We have examined signed copies of the
Registration Statement as filed with the Commission. We have also examined and
relied upon the minutes of meetings of the Board of Directors of the Company as
provided to us by the Company, record books of the Company as provided to us by
the Company, the Certificate of Incorporation and By-Laws of the Company, each
as restated and/or amended to date, and such other documents as we have deemed
necessary for purposes of rendering the opinions hereinafter set forth.

  In our examination of the foregoing documents, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to
us as originals, the conformity to original documents of all documents
submitted to us as copies, the authenticity of the originals of such latter
documents and the legal competence of all signatories to such documents.

  We assume that the appropriate action will be taken, prior to the issuance of
the Securities to register and qualify the Securities for sale under all
applicable state securities or "blue sky" laws.

  We express no opinion herein as to the laws of any state or jurisdiction
other than the state laws of the Commonwealth of Massachusetts, the Delaware
General Corporation Law statute and the federal laws of the United States of
America. To the extent that any other laws govern the matters as to which we
are opining herein, we have assumed that such laws are identical to the state
laws of the Commonwealth of Massachusetts, and we are expressing no opinion
herein as to whether such assumption is reasonable or correct.

  Based upon and subject to the foregoing, we are of the opinion that, subject
to the approval of the Certificate of Amendment to the Company's Restated
Certificate of Incorporation (in the form attached as Exhibit 3.5 to the
Registration Statement) by the stockholders of the Company and the filing of
such Certificate of Amendment with the Secretary of State of the State of
Delaware, the Securities to be issued by the Company will be duly authorized
for issuance and, when the Securities referred to in clause (ii) above are
issued against payment of the stipulated warrant exercise price, all of such
Securities will be validly issued, fully paid and nonassessable.

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  It is understood that this opinion is to be used only in connection with the
issuance of the Securities while the Registration Statement is in effect.

  Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters. This opinion
is based upon currently existing statutes, rules, regulations and judicial
decisions, and we disclaim any obligation to advise you of any change in any of
these sources of law or subsequent legal or factual developments which might
affect any matters or opinions set forth herein.

  We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act and to the use of our
name therein and in the related Prospectus under the caption "Legal Matters."
In giving such consent, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act or the
rules and regulations of the Commission.

                                          Very truly yours,

                                          /s/ Hale and Dorr LLP

                                          Hale and Dorr LLP

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