Black was seen as second-in-command to former president Jamie Dimon, who abruptly left the newly-merged company just a week ago.

Separately, Citigroup whose stock has been hampered recently by concerns about the integration of Citicorp and Travelers Group into Citigroup, said it was buying back $2 billion of its own stock.

Citigroup shares fell $1 to close at $45 in active trade on the New York Stock Exchange.

"It sounds like quite a few changes are being made in the top ranks at Citigroup, which is not that unusual," Diana Yates, an analyst at A.G. Edwards, said. She added that despite disruption at the top of the company, the share repurchase programme was a positive for the stock.

Black, 46, who joined Smith Barney in 1974, played a pivotal role in building the firm's equitiesbusiness.

His departure could foreshadow other resignations from leading Citigroup executives as the company struggles to put its different banking operations together, analysts said.

"There will probably will be others," James McDermott Jr., an analyst at Keefe, Bruyette & Woods Inc. said.

The changes in the upper ranks at Citigroup have heightened Wall Street's concern about the integration of Salomon Smith Barney with Citicorp's corporate bank, forcing the company to publicly defend its merger.

James Hanbury, an analyst at Schroders, said of the recent management changes at Citigroup: "Generally, I'm not that happy about them."

Last week, Citigroup announced plans to combine its corporate banking operations and named new leaders to replace Dimon and co-head of Salomon Smith Barney, Deryck Maughan.

Maughan, who once ran Salomon Brothers, was moved to the post of Citigroup vice-chairman in the recent changes, a title Wall Street sees as largely ceremonial.

After the unexpected shake-up, MichaelCarpenter, named to jointly lead the combined investment and corporate banks, asked the highly regarded Black to remain at the firm, a source familiar with the situation said on Monday.

But last Friday, Citigroup's co-chief executive, Sandy Weill, suggested that Black resign, the source said. Weill, who was once a mentor to Dimon, is also seen as responsible for making Dimon leave.

Robert DiFazio and Arthur Hyde, co-heads of global equities under Black, will now report directly to Carpenter, the company said. Carpenter ran Travelers life insurance and annuities businesses before being appointed to his new position.

Wall Street was shocked and disappointed by Dimon's sudden exit because it saw him as a skilled manager who was key to Citigroup's efforts to successfully meld its investment and corporate bank. It mourned his loss even though Salomon Smith Barney posted large losses in the third quarter that helped push down Citigroup's net income 53 per cent.

The integration of the investment andcorporate bank has not gone as smoothly as the combination of the company's different consumer businesses.