1
                                                                     EXHIBIT 5.1



                                  October 3, 2000

Global TeleSystems, Inc.
4121 Wilson Blvd.
8th Floor
Arlington, VA  22203

Ladies and Gentlemen:

     I have acted as counsel to Global TeleSystems, Inc., a Delaware Corporation
("GTS"), in connection with the combined Post-effective Amendment No. 1 to
Registration Statement on Form S-3 (no. 333-78097) and new Registration
Statement (No. 333-____)(collectively, the "Registration Statement") under the
Securities Act of 1933, as amended, relating to the offering from time to time
by certain holders of 10,000,000 Depositary Shares each representing 1/100 of a
share of 7 1/4% Cumulative Convertible Preferred Stock (liquidation preference
$50 per share) of GTS (the "Depositary Shares"), 100,000 shares of 7 1/4%
Cumulative Convertible Preferred Stock, par value $.0001 per share, (liquidation
preference $5,000 per share) of GTS (the "Convertible Preferred Stock"),
14,492,000 shares of common stock, par value $.10 per share ("Common Stock") of
GTS issuable on conversion of the Convertible Preferred Stock and 23,000,000
shares of Common Stock that maybe issued from time to time as dividends on the
Convertible Preferred Stock. The Depositary Shares, the Common Stock issuable on
conversion of the Convertible Preferred and the Convertible Preferred Stock are
being registered in connection with the obligation of GTS, pursuant to a certain
registration rights agreement, to register such securities held by certain
holders thereof that may be sold pursuant to the prospectus included in the
Registration Statement (the "Prospectus"). The Common Stock that may be issued
from time to time as dividends on the Convertible Preferred Stock is being
registered in connection with the obligation of GTS, pursuant to a certain
depositary agreement, to register such Common Stock. The Depositary Shares, the
Common Stock and the Convertible Preferred Stock are described in the
Prospectus. This opinion is an exhibit to the Registration Statement.

     In that capacity, I have reviewed the Registration Statement, the
Certificate of Incorporation, including the Certificate of Designations of the
Powers, Preferences and Relative Optional and Other Special Rights of 7 1/4%
Cumulative Convertible Preferred Stock and Qualifications, Limitations and
Restrictions Thereof (the "Certificate of Designations") and By-Laws of the
Company, the proceedings of the Board of Directors of the Company relating to
the aforementioned securities and originals, or copies certified or otherwise
identified to my satisfaction, of such 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:

                  (i) the Convertible Preferred Stock (which are represented by
Depositary Shares) has been duly authorized and is validly issued, fully paid
and non-assessable;

                  (ii) the Common Stock issuable on conversion of the
Convertible Preferred Stock has been duly authorized, and upon Conversion of the
Convertible Preferred Stock in accordance with the terms of the Certificate of
Designations, has been and will be validly issued, fully paid and
non-assessable; and

                  (iii) the Common Stock that may be issued from time to time by
the Company as dividends on the Convertible Preferred Stock, has been duly
authorized and, upon due declaration by the Board of




   2
Directors of the Company of a dividend on the Convertible Preferred Stock
payable in Common Stock, and due payment of such dividend in accordance with the
Certificate of Designations, has been and will be validly issued, fully paid and
non-assessable.

     My opinions expressed above are limited to Delaware corporate law and I do
not express any opinion herein concerning any other law.

     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,

                                          Arnold Y. Dean
                                          Deputy General Counsel