The justices today let stand a federal appeals court ruling that said traders could press their suit against the company as a class action. Pimco, manager of the world's largest bond fund, argued unsuccessfully that many of the investors in the class actually would have made money had the market manipulation taken place as alleged.

The 5-year-old lawsuit accuses Pimco of cornering the market for contracts on 10-year Treasury notes during May and June 2005. Pimco allegedly used its holdings to drive up the price for traders who had sold short -- that is, contracted to sell notes on a specified later date -- and needed to cover their positions.

In letting the suit go forward, the 7th U.S. Circuit Court of Appeals said that "although some of the class members probably were net gainers from the alleged manipulation, there is no reason at this stage to believe many were."

Pimco argued that the lower court ruling "invites ever-more adventurous class-action litigation."

The plaintiffs, Richard Hershey and Breakwater Trading LLC, say Pimco reaped $1 billion in profits from the scheme. Pimco allegedly bought as much as 87 percent of the contracts to buy June 2005 notes, or long positions.

Founded in 1971, Pimco had $1 trillion in assets under management as of Dec. 31, according to its Web site. Pimco, based in Newport Beach, California, is owned in part by Munich- based Allianz SE.

The case is Pacific Investment Management Company v. Hershey, 09-517.