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                                                                     EXHIBIT 5.1
 
                        [SHEARMAN & STERLING LETTERHEAD]
 
                                 March 24, 1999
 
Global TeleSystems Group, Inc.
1751 Pinnacle Drive
North Tower -- 12th Floor
McLean, Virginia 22102
 
Ladies and Gentlemen:
 
     We have acted as U.S. counsel to Global TeleSystems Group, Inc., a Delaware
corporation ("GTS"), in connection with the Registration Statement on Form S-3
(the "Registration Statement") being filed by GTS on the date hereof with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
with respect to 24,175,121 shares of common stock, par value $0.10 per share, of
GTS (the "GTS Common Stock"). The GTS Common Stock is being registered in
connection with the obligation of GTS, pursuant to certain registration rights
agreements and other undertakings, to register shares of GTS Common Stock held
by certain stockholders of GTS that may be sold pursuant to the prospectus
included in the Registration Statement (the "Prospectus"). The GTS Common Stock
is described in the Prospectus included in the Registration Statement. This
opinion is an exhibit to the Registration Statement.
 
     In that capacity, we 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 we
have deemed necessary or appropriate for purposes of this opinion. In such
examination, we have assumed the genuineness of all signatures and the
authenticity of all documents submitted to us as copies.
 
     Based upon the foregoing, we are of the opinion that the shares of GTS
Common Stock to which the Registration Statement relates have been duly
authorized and are validly issued, fully paid and non-assessable.
 
     Our opinions expressed above are limited to Delaware corporate law and we
do not express any opinion herein concerning any other law.
 
     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to us under the heading "LEGAL
MATTERS" contained in the Prospectus.
 
                                            Very truly yours,
 
                                                 /s/ SHEARMAN & STERLING
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