SAN FRANCISCO — California Attorney General Bill Lockyer sued nine fast-food giants yesterday, saying the law requires them to tell the public their potato chips and French fries contain a toxic chemical.

In a suit filed in Los Angeles County Superior Court, Lockyer sought a court order compelling the companies to place warnings on their cooked potato products because they contain higher levels of a suspected carcinogen, acrylamide, than other foods.

The defendants include Frito-Lay, KFC, McDonald's, Wendy's International and Procter & Gamble Distributing.

"I am not telling people to stop eating potato chips and French fries," Lockyer said in a statement. "I know from personal experience that, while these snacks may not be a necessary part of a healthy diet, they sure taste good."

But he said that Proposition 65, a ballot initiative passed by voters in 1986, requires businesses of all types to provide the public with "clear and reasonable" warnings before exposing them to potentially dangerous substances.

Representatives of the fast-food industry said Lockyer is unnecessarily alarming the public and unfairly singling out companies named in the suit.

"It is bound to misinform people if you have a warning on French fries but not on a potato," said Michele Corash, a San Francisco attorney who represents several of the defendants. "No one buys a potato to eat it raw. People will think if they make [French fries] on their own they will not have problems."

Lynn Markley, a spokeswoman for Frito-Lay, said, "There is no scientific evidence that the presence of acrylamide in food causes illness, and our Frito-Lay snacks are absolutely safe."

Acrylamide — known to cause cancer in the reproductive organs of laboratory animals exposed to high levels — has been on the state's list of carcinogens since 1990. But it was not known to be present in food until 2002, when scientists in Sweden discovered it in starchy foods cooked at high temperatures.

The lawsuit noted that the California Office of Environmental Health Hazard Assessment has estimated that consumers of French fries have as much as 125 times the amount of acrylamide that requires a warning under state law. Consumers of potato chips receive as much as 75 times the level requiring a warning.