Toivo Lahti grows papaya on the Big Island of Hawaii. Over the last few years, he watched other growers start planting trees that were genetically engineered to resist a devastating virus. But Mr. Lahti stuck to conventional varieties for his organic orchard, and thought it would remain free of biotechnology, which he opposes.

Then, last spring, some of Mr. Lahti's fruit tested positive for genetically modified seeds. "I was really surprised," Lahti said. "I didn't really know what was happening."

He cut down all 170 of his trees and is now replanting, without any guarantee that the same problem -- pollen from modified trees on other farms drifting on the wind to pollinate his trees -- won't happen again.

From papayas in Hawaii, to corn in Mexico and canola in Canada, the spread of pollen or seeds from genetically engineered plants is evolving from an abstract scientific worry into a significant practical problem.

Farmers, especially those raising organic crops, worry that they will lose sales from what they call contamination. Environmentalists worry that modified genes could escape from crops into weeds, wreaking ecological havoc. And once a gene has escaped, they say, there is no way to recall it.

Such concerns came to the fore last week when scientists at the Environmental Protection Agency reported that a type of creeping bent grass modified to resist Roundup, a popular herbicide, could pollinate conventional grass 13 miles away, much farther than previous studies had shown. That raised fears that the new gene could spread to wild grasses, creating weeds immune to the world's most widely used weed-killer.

Bioengineered crops seem to have a way of turning up where they are not wanted, through cross-pollination, intermingling of seed or other routes. StarLink corn, approved for animal feed but not for human consumption, ended up in taco shells and other groceries in 2000, prompting big recalls. Tiny amounts of corn engineered to produce a pharmaceutical got into 500,000 bushels of Nebraska soybeans. And engineered genes have apparently been detected in traditional varieties of corn growing in Mexico, the ancestral home of the crop and site of its greatest diversity, though the findings are disputed.

In light of these incidents, lawmakers, courts and the food industry are starting to consider how to ensure coexistence or determine liability. Even Sen. John Kerry, the Democratic presidential candidate, mentioned to Missouri farmers last month that he was considering an insurance plan to protect organic growers, according to The St. Louis Post-Dispatch. "If your crop gets polluted by a GMO crop, poof, you're gone," the newspaper quoted Kerry as saying, using the abbreviation for genetically modified organism.

Margaret Mellon, a biotechnology critic at the Union of Concerned Scientists, said liability "is like a huge slumbering giant out there hovering over the industry."

But the biotechnology industry and some scientists and lawyers say that the flow of genes from modified crops to other plants, while inevitable, will not be a big health, economic or legal problem.

For one thing, they say, genes have flowed naturally from crop to crop and from crop to weed for eons. "Since pollen flow has happened all along, you have to look and see if it's caused problems in the past," said Drew L. Kershen, a law professor at the University of Oklahoma. "The answer is no." He and others cite the example of two close relatives: canola, which is grown for vegetable oil, and oil-seed rape, which is grown for industrial lubricants and contains far higher levels of substances that can be harmful to people. The two can readily pollinate one another, but with proper buffers between them, they can be grown safely without intermingling, Kershen said.

Even if modified genes do flow between plants, some scientists say, so what?

One frequently mentioned concern is that an engineered gene for a trait like resistance to insects or drought will move from a crop to a weedy relative, yielding a superweed that could spread more widely. A herbicide-resistance gene is already known to have crossed over from canola to a wild mustard weed in Canada.

But the effect of adding a single gene to an existing weed is likely to be tiny compared with the effects of introducing a species into a new environment, like the natural but troublesome kudzu that has run amok in the South, according to C. Neal Stewart Jr., professor of plant molecular genetics at the University of Tennessee and author of Genetically Modified Planet: Environmental Impacts of Genetically Engineered Plants.

Stewart said that in his experiments, crossing insect-resistant bioengineered canola with weeds, the offspring were typically less fit than other weeds, because along with the insect resistance they also inherited other canola genes -- genes that are fine for the coddled life of a crop but unsuitable for the harsher life of a weed. "Gene flow is not this juggernaut evolutionary force that some people might make it out to be," he said.

Another concern relates to genes flowing from one crop to another. That could potentially create health problems if, say, corn engineered to produce pharmaceuticals turns up in cornflakes.

So far, the new traits introduced into commercial crops have mainly been insect or herbicide resistance, and have not been shown to be harmful. Still, some countries and some food companies do not accept them, whether because their safety has not been adequately proved, as matter of principle, or out of concern about consumer rejection. So farmers can lose sales from contamination.

In a 2002 survey by the Organic Farming Research Foundation, eight farmers reported losing organic certification from contact with genetically modified crops, and many more said they had to pay to test their crops.

But PG Economics, a British consulting firm, concluded in a report this year that organic and biotech farmers coexist very well. The report, financed partly by the biotechnology industry, found that the planting of organic corn and soy in the Midwest has surged since 1995, when bioengineered crops were introduced.

"Farmers need to cooperate with their neighbors, as they have done over hundreds of years," said Peter Barfoot, codirector of the consulting firm.

He and Kershen, the law professor, said that while organic standards do not allow farmers to use genetically modified plants, they allow room for accident. For instance, a crop can still be sold as organic even if traces of pesticide have drifted there from a nearby farm. Similarly, Barfoot and Kershen argued, organic farmers should not lose certification if some biotech pollen drifts over. Bob Scowcroft, executive director of the Organic Farming Research Foundation, said that while this might be true in theory, the rules are unclear, and some farmers are losing sales or certification because of even slight contamination.

Who should pay for damage or economic loss is in dispute. A group of organic canola farmers in Saskatchewan, Canada, is suing Monsanto and Bayer CropScience, saying that their introduction of genetically modified canola made it all but impossible to grow organic canola in the province. "All the organic farmers are doing is saying, take responsibility for your property," said Terry Zakreski, the lawyer for the farmers.

But Kershen said he thought the organic farmers would have trouble winning such cases. Seed producers are responsible for isolating their fields to protect the purity of their seed, he said, and organic farmers should do the same.

It will not be long before the issue of gene flow becomes even more complex. Researchers are working on genetically modified fish and insects. Plants, at least, do not usually swim or fly away.