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 Wednesday 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."

"It's very difficult to keep the food from contamination ... but I still say organic is a good idea," said Rodrigo Hurtado, a Washington physician.  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.

"The best thing consumers can do is to eat large amounts of fruits and vegetables," he said. "Pesticides allow these to be produced in more abundant manner, making them more affordable and offering consumers greater variety."

Some scientists also have raised concerns about the occurrence of mycotoxins on organic produce as well as the use of manure as fertilizer, which could carry harmful bacteria if not prepared properly. Mycotoxins, substances produced by fungi, can be prevented with the use of conventional pesticides.

The Agriculture Department data that were examined in the Consumers Union study showed residues on seven of 30 samples of organic fruit, and 22 of 97 samples of organic vegetables, or 23 percent of the total organic produce tested. Nine of 19 samples of organic spinach had pesticide traces, and four of 18 carrot samples.

By comparison, pesticides were found on 73 percent of the 26,571 samples of conventional foods that were tested.

"Less is better. Fewer residues and lower levels of residues are better than higher levels of residues and more residues," said Edward Groth, a senior scientist for Consumers Union, publisher of Consumer Reports.

The tests also included some samples of "green-labeled" foods - fruits and vegetables that are sold with claims of reduced pesticide use. There were pesticide residues on about half the samples of those products.

When the organochlorine chemicals were excluded from the analysis of organic foods, 13 percent showed positive for conventional pesticides.