DALLAS — Chalk up another remarkable feat for Michael Phelps: somehow making swimming trading cards popular.

An autographed 2004 trading card of the record-shattering Olympian was trading for as much as $500 today, just two weeks after industry experts say the collectible could be easily had for $25.

The market value could rise to $750 to $1,000 if Phelps breaks Mark Spitz's record of seven gold medals at a single Olympics this weekend, said Tracy Hackler, an associate publisher with Beckett Media LLP, a Dallas-based memorabilia company.

"It's unlike anything we've seen in the trading card category," said Hackler, whose company is an industry leader in collectibles pricing.

Arlington, Texas-based Donruss produced about 300 of the autographed Phelps cards in 2004 and about 5,000 "common" cards not bearing the American swimmer's autograph, Donruss spokesman Scott Prusha said.

The non-autographed cards have enjoyed a spike in market value too, going from $2.50 to about $20, Hackler said.

Phelps has won five gold medals, setting five world records in the process, at the Beijing Olympics. He picked up his 10th and 11th career gold medals Wednesday to become the winningest Olympian of all time.

The card was part of a special "Fans of the Game" insert by Donruss that paired celebrities with their favorite teams. Phelps' card includes him next to the Baltimore skyline and a pennant of his favorite NFL team, the Ravens.

Others cards in the set included actors John Travolta, James Gandolfini and Charlie Sheen. But none have gotten the response of Phelps' card.

Hackler said Beckett listed the card at $150 yesterday and $350 today, when he said it was trading online for upward of $500.

"We're kind of in uncharted waters here," Hackler said. "No pun intended."