On Fla. Menus, a Favorite Fish Experiences Identity Theft

The most common substitute was Asian catfish, he said. Emperor fish, which are known to swim with grouper, was next.

Connell said the restaurant bought the grouper from Sysco, a large national food-service company.

"They're the industry standard -- the best," he said.

Sysco West Coast Florida representatives said that they know a problem exists, and that last year they began testing about one of every several hundred to several thousand pounds of purported grouper. Each test costs $180.

They have also warned suppliers that they will not do business with those who falsely label their products.

Since testing began, the proportion of false grouper has dropped from 50 percent to 20 percent, said Lee Ann Applewhite, the chief executive of Applied Food Technologies, which conducted the tests for Sysco.

Still, fish that are not grouper can slip through.

"It's a classic case," said Spaeth, "of let the buyer beware."