he world's most widely grown genetically engineered crop contains some unexpected DNA next to its inserted gene, casting some doubts on the biotechnology industry's assertions that its technology is precise and predictable.

The mysterious DNA was found in the Monsanto Company's Roundup Ready soybeans by Belgian government and university scientists, who described their findings in a paper published yesterday in the journal European Food Research and Technology. Greenpeace called yesterday for countries to re-evaluate the regulatory approvals of the soybeans, saying that Monsanto did not know as much as it should about its product. The unknown DNA could possibly affect the safety of the beans, the group said.

"I don't think you can come out and say it's unsafe," said Dr. Janet Cotter-Howells, a scientist for Greenpeace in Britain. "You can just say it's unknown whether it's unsafe or not."

Monsanto acknowledged that the extra DNA was there, but it said it was confident that the soybean was safe and that the unknown DNA had no effect on the plant. Dr. Jerry J. Hjelle, the company's vice president for regulatory affairs, said the DNA segment had been in the crop since the beginning as it went through testing to prove its safety.

Products made from Roundup Ready soybeans have been eaten by people and animals for five years with no reports of health problems. Still, the findings could cause some embarrassment for Monsanto and the agricultural biotech industry because they raise questions about how well the molecular makeup of the products is characterized.

Roundup Ready soybeans contain a gene from a bacterium that allows the plants to withstand Monsanto's Roundup herbicide. Farmers can thus spray their fields with Roundup throughout the growing season to kill weeds without harming the crop. More than half the soybeans grown in the United States are now Roundup Ready. In Europe and Japan the beans are approved for use but not for planting.

This is the second time that scientists have found something in Roundup Ready soybeans that Monsanto did not seem to know was there and had not cited at the time of the product's approval.

Last year the Belgian scientists and Monsanto, working independently, found that the soybeans contained not only one complete copy of the bacterial gene, as intended, but two fragments of that gene. Monsanto filed reports with regulators around the world offering data to show that the fragments were not active genes and had no effect on the plant.

The paper now being published contains another revelation. Adjacent to one of those gene fragments is another stretch of DNA that Monsanto, in its report to regulators last year, had assumed was the soybean's native DNA.

But the Belgian scientists, led by Dr. Marc De Loose of the Center for Agricultural Research in Melle, said they could not find this stretch of DNA in the soybean that had not been genetically engineered.

They suggested that this unknown DNA is probably the plant's own DNA but that it was somehow rearranged, or scrambled, at the time the bacterial gene was inserted. Another possibility, they said, is that a portion of the plant's DNA was deleted, leaving other DNA in that position.

Dr. Hjelle, of Monsanto, said that the new paper by the Belgian scientists had been available online for some time and that Monsanto had already discussed the information with regulators. He said the unexpected DNA had been found because more sensitive techniques had made it practical for the first time to determine the sequence of the DNA flanking the inserted gene. "As methods improve," he said, "we can find things from a detailed perspective that we couldn't 10 years ago."

Dr. Hjelle said the unknown sequence was only 534 letters long out of a soybean genome of about 1.5 billion letters and was not meaningful. He also said that the jumbling up of DNA near the spot where a new gene was inserted was "expected by people who understand the science."

Dr. David Ow, a senior scientist at the Department of Agriculture's Plant Gene Expression Center in Albany, Calif., said that an inserted gene did not always integrate itself into a plant in a neat way.

"It's not so much that rearrangements occur, but what are the consequences of it?" he said. Dr. Ow said he did not think that this would pose a public safety issue, but he said it would pose a public perception problem for the industry.

"If one is submitting a product it has to be characterized to the extent required by the regulatory bodies," he said.