Environmentalists said the government's decision will help prevent biotech corn from contaminating native varieties in Mexico, the birthplace of corn and still a storehouse of genetically valuable native species.

But the decision, announced late Monday by Mexico's Agriculture Department, angered some biotech supporters that said it would limit access to plants that could reduce pesticide and herbicide use and have other advantages for local farmers. Columnist Sergio Sarmiento, writing in the newspaper Reforma on Wednesday, called it "cowardly."

Genetically modified corn "is already in use in many parts of the world and it has enormous benefits, both in terms of the environment and production, given that it reduces pesticide use," Sarmiento wrote.

Even environmentalists don't think Monday's decision is the last word.

"This is temporary, because there is so much pressure from the multinationals," said Gustavo Ampugnani of Greenpeace Mexico. "They are going to put a lot of pressure on the incoming administration" of president-elect Felipe Calderon, who takes office Dec. 1.

Monday's decision turned down all seven requests filed by companies including St. Louis-based Monsanto, Wilmington, Del.-based DuPont Co.'s Pioneer Hi-Bred International Inc. subsidiary, and others.

"We were surprised by this decision," said Eduardo Perez Pico, director of technological development at Monsanto's Mexico subsidiary, which had applied to start experimental fields in the northern states of Sinaloa, Sonora, and Tamaulipas.

"These are not centers of origin or biodiversity of corn," Perez Pico said, referring to the areas where corn ancestor plants or primitive varieties grow naturally.

Under current law, such areas are off-limits to biotech planting, in part to protect the genetic traits of those ancestor varieties in case their traits are needed for hybridization efforts in the future.

In areas of Mexico where corn is determined to be a non-native or non-original crop, "there is the possibility of a permit being granted for the first phases of experimental projects," said Pedro Mata, of Mexico's food safety agency.

Mata said Monday's ruling hinged on an ongoing debate over whether any area of Mexican can be designated as a non-origin region for corn.

"The researchers and experts are still discussing it, and there are some controversies," Mata said. There is no deadline for drawing up the map of "safe" areas.

Mexico imposed a moratorium on the planting of genetically modified crops in 1998, but in 2005, President Vicente Fox signed a bill that set out a framework for approving such planting in the future.

Farmers in Mexico first bred corn some 6,000 to 8,000 years ago. The country is home to at least 59 species of maize, from the protein-rich variety used to make tortilla chips to a softer grain mashed for use in tamales.

A study in the Sierra de Juarez region in the southern state of Oaxaca found evidence of transgenic corn contamination in 2000 from corn that was apparently imported for food use. The study was published and then retracted by the science journal Nature.

Another study by Mexican and U.S. researchers in 2004 found no trace of genetically altered corn in crops in the same area four years later.