In a finding that has taken researchers by surprise and alarmed environmentalists, the Mexican government has discovered that some of the country's native corn varieties have been contaminated with genetically engineered DNA.

The contaminated seeds were collected from a region considered to be the world's center of diversity for corn — exactly the kind of repository of genetic variation that environmentalists and many scientists had hoped to protect from contamination. The result was unexpected because genetically modified corn, the presumed source of the foreign genes, has not been approved for commercial planting in Mexico.

Scientists expressed concern that the foreign genes could act to reduce genetic diversity in the country's native corn varieties and in the wild progenitor of domesticated corn, known as teosinte. If any of the foreign genes are very advantageous, plants carrying those genes could begin to dominate the population. In such cases genetic variation will be lost as the diversity of plants not carrying the foreign genes decreases or disappears. Whether that will happen or has happened remains unknown.

In addition to being one of the world's most important crops, corn is viewed with a near religious reverence in Mexico, with seeds of native varieties passed down from generation to generation. Until now, scientists said researchers had assumed that these varieties, some of which are grown only by subsistence farmers in remote areas, were pristine.

"These are the extremes, the places where you would really not expect to find contamination," said Dr. Ignacio Chapela, a microbial ecologist at the University of California at Berkeley, saying the results are an indication of widespread contamination. "The only reason they found it there is because that's the only place they've looked."

Scientists said the results also indicated that crop genes might be able to spread across geographic areas and varieties more quickly than researchers had guessed.

"It shows in today's modern world how rapidly genetic material can move from one place to another," said Dr. Norman C. Ellstrand, evolutionary biologist at University of California at Riverside. He said the real worry was that other foreign genes — like pharmaceutical-producing genes being developed in crops — could also find their way quickly and unnoticed into distant food sources.

Genetically engineered corn, known as Bt corn because it produces the insecticide known as Bt, has been in use by farmers in the United States since 1996.

Mexico's Ministry of the Environment and Natural Resources made the announcement on Sept. 18 that contaminated corn had been found in 15 different localities. The announcement credited Dr. Chapela with the initial discovery but described only the results from government-led research. Neither Dr. Chapela's team nor the Mexican teams' work has yet been published.

Scientists assume the native corn became contaminated through interbreeding with Bt corn, but how Bt corn may have come to be planted in Mexico remains a matter of speculation. While not approved for planting, biotech corn is legally imported into Mexico for use in food. Greenpeace, calling the contamination a form of genetic pollution, is calling on Mexico to ban all importation of genetically modified corn.

The Mexican government has not disclosed exactly what genes were found. Exequiel Ezcurra, the director of the National Institute of Ecology, which worked on the study, did not respond to requests for an interview. But Dr. Chapela, who is familiar with the Mexican work, said the researchers had identified the presence of DNA sequences from the cauliflower mosaic virus. This DNA is used nearly universally in genetically engineered plants and does not produce Bt insecticide.

As a result, it is still unclear whether any of the contaminated corn has the ability to produce the Bt insecticide.

Scientists may eventually be able to quantify the biological effects of the contamination, but some say the cultural cost in a country where corn is a symbol of the Mexican people may be harder to measure.

"The people are corn," said Dr. Chapela, who is Mexican, "and the corn is the people."

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