With Charles Prince resigning as chairman and chief executive of Citigroup and the bank poised to announce a write-off of at least $8 billion more on dud mortgage securities, the question now is, How will new leaders break up the financial supermarket that former chairman and C.E.O. Sanford I. Weill built, and how soon?

Citigroup, the nation's biggest bank by assets, said over the weekend that Robert E. Rubin would step in as chairman and Sir Win Bischoff would move over from Europe to act as acting chief executive. Bischoff is chairman of Citigroup's European operations.

The bank's board has also named a special committee to find a new C.E.O. Rubin will be on the committee, which will be headed by Time Warner C.E.O. Richard D. Parsons, and will include Alain J.P. Belda and Franklin A. Thomas.

Heavy losses in subprime mortgage-backed securities are driving the changes. Citi is expected to announce a write-down of $8 billion to $12 billion in its mortgage assets this week. That would be in addition to the $5.9 billion write-down announced in October.

Seeking to insulate the rest of the bank from these problem assets, Rubin said the bank has created a new business unit, "the sole focus of which will be on managing the assets related to subprime mortgage securities and their resultant exposures." The unit "will be separate from the other parts of our capital markets and banking business," Rubin added.

Breaking Citigroup into more manageable separate units is just what many investors are seeking. They contend that the conglomerate built by Weill has become a conflicted, sluggish behemoth trying to do too much in too many fields—a retail bank, a brokerage, an investment bank, a mortgage lender, and a credit card company.

Weill's theory in building Citigroup, which also one had a big insurance component, was that varied businesses would shield the firm from a slump in any one unit and that each operation could "cross-sell" the products and services of others.

But it has also exposed the bank to the compounded effects of problems in several of its businesses. Bad bets by the investment bank, for example, have come atop a slump in mortgages and credit cards. The company was so big and unwieldy, critics say, that Citigroup's top managers could not anticipate and manage risks adequately.

Also on Portfolio.com:

• A Legend's Bloated Legacy
• Is Citigroup Too Big?
• Superfund for Banks
• Credit Crunched