Exhibit 5

                                BAKER & McKENZIE
                                Attorneys at Law

                            2300 Trammell Crow Center
                                2001 Ross Avenue
                               Dallas, Texas 75201


                                January 10, 2001


Board of Directors
Transaction Systems Architects, Inc.
224 South 108th Avenue
Omaha, Nebraska  68154

Re:      Transaction Systems Architects, Inc. (the "Company")

Gentlemen:

         We have acted as your counsel in connection with the  registration,  on
a Registration  Statement on Form S-8 (the  "Registration  Statement") under the
Securities Act of 1933, as amended,  of 370,000 shares of the Company's  Class A
Common  Stock,  $.005  par value per  share  (the  "Stock").  The Stock is to be
issued by the Company upon the exercise of  outstanding  options  granted  under
the  MessagingDirect  Ltd. Amended and Restated  Employee Share Option Plan (the
"Plan") and to be assumed by the Company  pursuant  to a  Combination  Agreement
dated as of October 24,  2000,  by and among the  Company,  Transaction  Systems
Architects  Nova Scotia  Company,  TSA  Exchangeco  Limited and  MessagingDirect
Ltd. (the  "Combination  Agreement")  and Plan of Arrangement  attached  thereto
(the "Plan of Arrangement").  We have reviewed the Registration  Statement,  the
Combination Agreement,  the Plan of Arrangement,  the charter and by-laws of the
Company,  corporate  proceedings  of the  Board  of  Directors  relating  to the
issuance of the shares of Stock,  and such other  documents,  corporate  records
and  questions  of laws as we have  deemed  necessary  to the  rendering  of the
opinions expressed below.

         In all  examinations  of documents,  instruments  and other papers,  we
have  assumed  the  genuineness  of all  signatures  on original  and  certified
documents and the  conformity to original and certified  documents of all copies
submitted to us as  conformed,  photostatic  or other  copies.  As to matters of
fact  which  have  not  been  independently  established,  we have  relied  upon
representations of officers of the Company.

         Based  upon the  foregoing,  we are of the  opinion  that  the  370,000
shares of Stock to be issued by the  Company,  as  described in the Plan and the
Plan of  Arrangement,  will be validly  issued,  fully  paid and  non-assessable
when  issued  and paid for in the  manner  contemplated  in the Plan and Plan of
Arrangement.

         The opinion  expressed above is limited to the General  Corporation Law
of the State of Delaware and the federal laws of the United States of America.

         This  opinion  letter may be filed as an  exhibit  to the  Registration
Statement.  In giving this  consent,  this firm does not  thereby  admit that it
comes within the category of persons whose  consent is required  under Section 7
of the Securities Act of 1933, as amended,  or the rules and  regulations of the
Securities and Exchange Commission promulgated thereunder.



                                            Very truly yours,



                                            Baker & McKenzie