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                                                                     EXHIBIT 5.1

                   [GLOBAL TELESYSTEMS GROUP, INC. LETTERHEAD]

                                 April   , 2000

Global TeleSystems Group, Inc.
4121 Wilson Boulevard
8th Floor
Arlington, Virginia  22203

Ladies and Gentlemen:

               I am Deputy General Counsel to Global TeleSystems Group, Inc., a
Delaware corporation ("GTS"), and have advised GTS in connection with the
preparation and filing with the Securities and Exchange Commission under the
Securities Act of 1933, as amended, the Registration Statement on Form S-3 (the
"Registration Statement") of GTS with respect to 4,709,695 shares of common
stock, par value $0.10 per share, of GTS ("Common Stock"). Of such 4,709,695
shares of Common Stock, (i) 4,440,263 shares have been, or will be, issued by
GTS in connection with its acquisition of Netcom Internet Limited ("Netcom"),
(ii) 254,432 shares will be issued by GTS in connection with stock options
granted to Netcom employees and (iii) 15,000 shares will be issued by GTS
pursuant to a stock option agreement between GTS and a former consultant (the
shares described in the foregoing clauses (i), (ii) and (iii), being
collectively referred to as the "Shares"). The Common Stock is described in the
prospectus (the "Prospectus") included in the Registration Statement. This
opinion is an exhibit to the Registration Statement.

               In that capacity, I have reviewed the Registration Statement and
originals, or copies certified or otherwise identified to our satisfaction, of
other documents, corporate records, certificates and other instruments as I have
deemed necessary or appropriate for purposes of this opinion. In such
examination, I have assumed the genuineness of all signatures and the
authenticity of all documents submitted to me as copies.

               Based upon the foregoing, I am of the opinion that the Shares
have been, or when issued will be, duly authorized, validly issued, fully paid
and non-assessable.

               I hereby consent to the filing of this opinion as an exhibit to
the Registration Statement and to the reference to me under the heading "LEGAL
MATTERS" contained in the Prospectus.

                                                   Very truly yours,

                                                   /s/ Arnold Y. Dean

                                                   Arnold Y. Dean
                                                   Deputy General Counsel