Exhibit 5.1

                            [MINTZ LEVIN LETTERHEAD]

                   New York Boston Washington Reston New Haven


                                                                January 24, 2005


deltathree, Inc.
75 Broad Street, 31st Floor
New York, New York 10004


Ladies and Gentlemen:

We have acted as counsel to deltathree, Inc., a Delaware corporation (the
"Company"), in connection with the preparation and filing with the Securities
and Exchange Commission of a Registration Statement on Form S-8 (the
"Registration Statement"), pursuant to which the Company is registering the
issuance under the Securities Act of 1933, as amended, of a total of 1,110,948
shares (the "Shares") of its Class A common stock, $.001 par value per share
(the "Common Stock") This opinion is being rendered in connection with the
filing of the Registration Statement. All capitalized terms used herein and not
otherwise defined shall have the respective meanings given to them in the
Registration Statement.

In connection with this opinion, we have examined the Company's Certificate of
Incorporation, as amended, and Amended and Restated By-Laws, both as currently
in effect; such other records of the corporate proceedings of the Company and
certificates of the Company's officers as we have deemed relevant; and the
Registration Statement and the exhibits thereto.

In our examination, we have assumed the genuineness of all signatures, the legal
capacity of natural persons, the authenticity of all documents submitted to us
as originals, the conformity to original documents of all documents submitted to
us as certified or photostatic copies and the authenticity of the originals of
such copies.

Based upon the foregoing, we are of the opinion that (i) the Shares have been
duly and validly authorized by the Company and (ii) the Shares, when sold, will
have been duly and validly issued, fully paid and non-assessable shares of the
Common Stock, free of preemptive rights.

Our opinion is limited to the applicable provisions of the Delaware
Constitution, the General Corporation Laws of the State of Delaware and the
reported judicial decisions interpreting those laws, and we express no opinion
with respect to the laws of any other jurisdiction. No opinion is expressed
herein with respect to the qualification of the Shares under the securities or
blue sky laws of any state or any foreign jurisdiction.



                  New York Boston Washington Reston New Haven



MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.

January 24, 2005
Page 2


We understand that you wish to file this opinion as an exhibit to the
Registration Statement, and we hereby consent thereto. We hereby further consent
to the reference to us under the caption "Legal Matters" in the prospectus
included in the Registration Statement.


                                               Very truly yours,

                                               /s/ Mintz, Levin, Cohn, Ferris,
                                               Glovsky and Popeo, P.C.


                                               Mintz, Levin, Cohn, Ferris,
                                               Glovsky and Popeo, P.C.