Just days ago, E. Stanley O’Neal “retired” as CEO of Merrill Lynch in the aftermath of $8.5 billion in write-downs. Now with Citigroup facing the aftermath of $11 billion in write-downs, its CEO, Charles Prince has “retired.”

It’s official. Two major U.S. banks are without chief executive officers, days before filing quarterly financial statements with federal regulators, all because of ongoing concerns about their exposures to credit derivatives.

Citigroup (nyse: C – news – people )’s Charles Prince, under pressure for months because of the bank’s lackluster performance, quit Sunday. Former U.S. Treasury Secretary Robert E. Rubin, chairman of the executive committee of Citi and a member of the board of directors, will serve as chairman of the board. Sir Win Bischoff, chairman of Citi Europe, will serve as acting chief executive officer. The company said it would move to fill the position “as expeditiously as possible.”

More stunning: the company also announced massive write-downs, $8 billion to $11 billion, related to its subprime mortgage assets following downgrades of the debt by ratings agencies.