Reuters reports that Citigroup Inc. (NYSE: C) has accepted the resignation of Chuck Prince five days after Merrill Lynch & Co.'s (NYSE: MER) CEO Stanley O'Neal quit. And, as expected, Robert Rubin, 69, has taken the Chairman's job while the Sir Win Bischoff, 66, who runs Citigroup's European operations, will become acting CEO.

The big shocker was not in Citigroup's organization -- it was the magnitude of the additional write off of bad mortgage securities -- in the range of $8 billion to $11 billion. This is on top of the $6.5 billion that Citigroup wrote off three weeks ago for subprime mortgages, loan losses and other debt. I shudder to think how big a write off Citigroup will have to take if it can't get a bailout for its $80 billion worth of Structured Investment Vehicles.

How will a permanent CEO be selected? A special board committee consisting of Rubin, Alain Belda, Richard Parsons and Franklin Thomas will seek a replacement for Prince from internal and external candidates. As I posted this morning, I vote for Thain.

Update: Bloomberg News reports that Citigroup is trading up 5.8% in Tokyo where Monday trading has begun on the announcement of Prince's resignation.

Peter Cohan is President of Peter S. Cohan & Associates. He also teaches management at Babson College and edits The Cohan Letter. He owns Citigroup stock and has no financial interest in the other securities mentioned.