Form 6-K

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                        Report of Foreign Private Issuer

                        Pursuant to Rule 13a-16 or 15d-16
                     of the Securities Exchange Act of 1934

                          For the month of August, 2005

                              CONVERIUM HOLDING AG
                 (Translation of registrant's name into English)

                                 Baarerstrasse 8
                                   CH-6300 Zug
                                   Switzerland
                    (Address of principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports
under cover Form 20-F or Form 40-F.

                          Form 20-F X      Form 40-F
                                   ---              ---

Indicate by check mark whether the registrant by furnishing the information
contained in this Form is also thereby furnishing the information to the
Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

                               Yes              No X   
                                  ---             ---

If "Yes" is marked, indicate the file number assigned to the registrant in
connection with Rule 12g3-2(b): 82- Not Applicable




Zurich Cantonal Bank increases its holding of Converium's registered shares to
5.87% 
Disclosure of shareholdings pursuant to Art. 20 SESTA

Zug, Switzerland - August 23, 2005 - Converium Holding Ltd has been notified
that Zurich Cantonal Bank, Tessinerplatz 7, 8002 Zurich, Switzerland, increased
its holding of registered shares of Converium Holding Ltd, Zug on August 18,
2005 to 5.87% or 8,613,148 registered shares with voting rights.

Zurich Cantonal Bank states that the above-mentioned stock of shares is a pure
trading position, which is generated during the normal course of its derivative
business.

Contacts:

Zuzana Drozd
Head of Investor Relations
zuzana.drozd@converium.com
Phone    +41 (0) 44 639 91 20
Fax      +41 (0) 44 639 71 20


Esther Gerster
Head of Public Relations
esther.gerster@converium.com
Phone    +41 (0) 44 639 90 22
Fax      +41 (0) 44 639 70 22


www.converium.com




                                   SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned, thereunto duly authorized.


                                                CONVERIUM HOLDING AG


                                                By: /s/ Terry Clarke
                                                    Name:  Terry Clarke
                                                    Title: CEO


                                                By: /s/ Christian Felderer
                                                    Name:  Christian Felderer
                                                    Title: General Legal Counsel


Date: August 23, 2005