Washington ? Think organic fruits and vegetables are free of pesticides? Think again.

Almost one-fourth of the organic produce in grocery stores could contain traces of pesticides, including long-banned chemicals like DDT, scientists say.

A Consumers Union-led study of government-collected data found pesticide residue on 23 percent of organic fruits and vegetables and on nearly 75 percent of conventionally grown produce.

The findings don’t mean that any of the produce is unsafe. The residues are seldom even close to the limits set by the Environmental Protection Agency.

“Consumers who seek to reduce their exposure to pesticide residues can do so reliably by choosing organic produce,” the scientists wrote. “However, none of the choices available on the market is completely free of pesticide residues.”

The study is being published today in the journal Food Additives and Contaminants.

Much of the residues found in organic crops were of organochlorine pesticides, chemicals including DDT and chlordane that plants can soak up from the soil decades after the products were used. Other chemicals could have been applied to the crops improperly or drifted onto the organic fields from adjacent farms, the scientists said.

One sample of organic peaches contained 3.3 parts per million of the pesticide phosmet, suggesting the crop was sprayed shortly before harvest, the study said.

“You normally think that organic are the ones without the pesticides,” said Vicki Kirkbride, an Arlington, Va., executive shopping Tuesday at a Fresh Fields supermarket. A sign in the store said organic foods were “grown without the use of synthetic pesticides, fungicides or fertilizers.”

Although organic crops account for just 2 percent of U.S. fruit and vegetable acreage, the industry has been growing rapidly. Sales of organic foods reached $7.8 billion in 2000, a 20 percent increase from the year earlier, according to Packaged Facts, a market research firm.

The study was based on sampling by the Agriculture Department and the state of California as well as by the scientists themselves. It did not take into account the many special pesticides that are approved for organic crops, including sulfur and bacteria sprays.

Those products are generally considered less toxic than pesticides used by conventional farms and government inspectors do not test for them. However, one natural pesticide used by organic farmers, pyrethrum, may cause cancer, and another is linked to neurotoxic effects in rats. The study called for more research on those pesticides.

“Consumers need to recognize that organic production doesn’t mean pesticide-free production,” said Carl Winter, a food toxicologist at the University of California, Davis.