A JPMorgan Chase & Co. trader of derivatives linked to the financial health of corporations has amassed positions so large that he's driving price moves in the $10 trillion market, traders outside the firm said.

The trader is London-based Bruno Iksil, according to five counterparts at hedge funds and rival banks who requested anonymity because they're not authorized to discuss the transactions. He specializes in credit-derivative indexes, a market that during the past decade has overtaken corporate bonds to become the biggest forum for investors betting on the likelihood of company defaults.

Investors complain that Iksil's trades may be distorting prices, affecting bondholders who use the instruments to hedge hundreds of billions of dollars of fixed-income holdings. Analysts and economists also use the indexes to help gauge perceptions of risk in credit markets.