Junk bond underwriters affiliated with U.S. banks led 32% of the new issues in the first quarter, about the same as a year ago. But there was a sharp divergence between banking companies that cater to big-cap companies and those that focus on small or mid-cap clients.

"We are seeing a tiering in market share, with a few larger participants separated from everyone else," said Randolph Barker, Salomon Smith Barney's co-head of global debt capital markets and syndicate.

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