Mark Harris, chief of the department's crop statistics branch, said Wednesday that "probably there are some individuals who may not have understood the rules and didn't follow their contracts precisely.''

The survey looked at 289,640 farms in 10 Midwestern states – Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Nebraska, Ohio, South Dakota and Wisconsin – to see how many were growing the biotech corn variety, Bt. It found that 93,530 farms, or 32 percent, were growing 4.2 million acres of Bt corn.

Of those, only about four-fifths were complying with an Environmental Protection Agency requirement that farmers grow Bt corn in fields surrounded with conventional corn. This perimeter is meant to be a refuge to prevent pests from developing resistance to the Bt variety.

EPA spokesman Dave Deegan said the agency was still reviewing the survey. "We've not made any conclusions at this point,'' he said.

The Agriculture Department estimates that 17 million acres of Bt corn were planted nationwide this year.

Bt corn is genetically engineered to contain a gene from a bacterium, bacillus thuringiensis, that can kill corn borers, a common field pest. Because the gene behaves like a pesticide, EPA is charged with regulating the plants.

However, agency officials do not visit Bt crops to check and see if farmers are complying. Instead, EPA relies on seed companies to ensure that the farmers know they must plant the refuge.

The Center for Science in the Public Interest, said EPA needs to implement a system that would give companies an incentive to ensure farmers to follow the rules.

"They don't have an incentive to penalize and fine noncompliant farmers because those farmers are their customers,'' said Gregory Jaffe, the center's director for biotechnology.

He suggested that EPA restrict companies from selling Bt seeds in counties where many farmers have failed to comply with the planting requirements. Jaffe added that farmers also need an incentive to plant conventional corn around Bt crops.

Now, farmers who don't comply are supposed to get a letter from the biotech seed company. If they still fail to comply, the company can stop selling them seed.

Sharon Troughton, a spokeswoman for Monsanto Co., a biotech firm, said companies are intensifying their efforts to educate farmers about how to grow Bt corn. The firms that sell Bt seed -- Monsanto, Dow AgroSciences, Pioneer Hi-Bred International, and Syngenta -- also are conducting annual surveys to check on farmers.

Troughton said it's a good sign that the majority of farmers are aware of the government regulations, but "obviously we want 100 percent compliance, and we're working towards that.''

On the Net: USDA National Agricultural Statistics Service: http://www.usda.gov/nass