The wheat, produced by the biotechnology giant Monsanto, has been spliced with a gene that protects it from Monsanto's powerful and popular herbicide Roundup, allowing farmers to kill weeds efficiently without harming their crop. Monsanto says it will be ready for farmers within two to four years, and the company estimates it will increase crop yields by $6 to $11 an acre.

The company hopes the wheat will also lead to other engineered improvements to one of the world's oldest and most important crops, but the international reaction illustrates just how contentious and unpredictable genetically engineered crops have become.

As news of Monsanto's wheat has spread, buyers from Japan to Europe and Egypt have told U.S. exporters that their consumers will not accept genetically modified wheat because of general fears about possible harm to the environment and human health from engineered crops. Some have said that the wheat's very presence on American farms could threaten future purchases of all U.S. wheat. Half of all American wheat is exported, accounting for $3.7 billion in sales and almost 20 percent of all agricultural commodities shipped abroad in 1999.

"We may in the future have a biotech wheat that the world does want," said Darrell Hanavan, chairman of a joint wheat industry committee on biotechnology. "But we need to proceed now under the assumption that some markets won't want it anytime soon. And the challenge will be to make sure that buyers and their customers get exactly what they want."

In an effort to respond to these concerns, Monsanto has agreed to an unprecedented wheat industry request to put in place a system to strictly segregate the modified wheat before it is ever sold to farmers or even approved by regulators. The company has also agreed generally to promote wheat biotechnology to buyers and consumers abroad.

"Some farmers do have concerns about the market for our wheat, but many really want it," said Monsanto spokesman Mark Buckingham. "Farmers need to make improvements and reduce costs, and farmers know our technology can provide that . . . We want to be frank and open because in the current atmosphere, it's very easy for misconceptions to arise."

About 55 percent of U.S. soybeans and 25 percent of corn harvested last year were genetically engineered. Development of genetically modified wheat has lagged behind other crops because it is a more complex plant, made from the union of three wild grasses that have been improved by farmers over the millennia. Rights to wheat varieties are often publicly owned, which can make them less desirable to profit-making companies.

Since last year's Starlink corn debacle -- in which an engineered corn only approved for animal consumption inadvertently made it into the human food supply -- already negative attitudes in major foreign markets about genetically modifed foods have intensified.

The result is that unlike the American corn and soybean industries, which quickly embraced biotech products in the mid-1990s, many in the wheat industry are approaching biotechnology now more as a challenge to surmount than an immediate opportunity to exploit. That wheat has an unusual emotional resonance for many people stemming from its use in bread, the ancient "staff of life," just adds to the challenge.

"Monsanto's wheat can definitely be a real benefit to the producers and our country," said Phil Isaak, a board member of U.S. Wheat Associates, the national organization that promotes American wheat exports for growers. "But unless we get worldwide public approval of it, we have to take the position of resisting release for commercialization."

Critics of biotechnology call the worldwide debate over genetically modified wheat a positive development, and are pleased it is happening well before the crop is actually introduced. While major U.S. scientific organizations have generally found that current genetically engineered crops pose no danger to the environment or human health, opponents argue that taking genes from one kind of plant or animal and inserting it into another could have unforeseen long-term consequences.

"It is a very healthy thing for people to be asking now if we really need this wheat, if it's wise to release it and whether it will benefit people who need help," said Margaret Mellon of the Union of Concerned Scientists. "This has never happened before with a major product of biotechnology."

Monsanto's wheat is being tested in greenhouses in the upper Midwest and bred into local varieties. Company officials say they are in no rush to introduce Roundup Ready wheat, and will bring it onto the market gradually when they do. The company has asked for Environmental Protection Agency approval to add wheat to the approved list of crops for its Roundup herbicide, but has not yet approached two other federal agencies.

Industry and company officials said they hoped to devise a segregation system for engineered wheat -- which would parallel those already in place for some special conventional varieties -- by year's end.

Montana wheat farmer Frank Elling said he would be happy to use Roundup Ready wheat if he was certain customers would accept it. But his Pacific Rim buyers have made their reservations known, and Asian governments have taken dramatic steps in recent years to reject shipments of genetically modified crops.

Japanese officials, for instance, turned back a boatload of corn last year suspected to contain the Starlink variety, and Thai officials did the same with a shipment of wheat 18 months ago. In that case, officials concluded that the American wheat had been mixed with small amounts of engineered corn while being transported from the West Coast.

Similar messages of concern have been coming in to the 17 international offices of U.S. Wheat Associates, the American export marketing group. A letter from Tsutoma Shigeta of the Japan Flour Millers Association said, for instance, that "Japanese consumers are highly suspicious and skeptical about safety of [genetically modified] farm products which may be hazardous to human health and environment. Under the circumstances, I strongly doubt that any bakery and noodle products made of [modified] wheat or even conventional wheat that may contain [modified] wheat will be accepted in the Japanese market."

Jef Smidts of the Dutch wheat supplier Andre & Cie wrote even more bluntly, "[Genetically modified] wheat for sure will be a market destructor." Because of such concerns, legislators in Montana and North Dakota have introduced bills to place a moratorium on the use of genetically engineered wheat.

Val Giddings, vice president for food and agriculture for the Biotechnology Industry Organization, said he has heard similar concerns, but that he believes the "perception of resistance is substantially greater than the reality is likely to be.

"Monsanto has recognized and is acting on the understanding that some folks want to have more input into this product," he said. "They are trying to do this in an open and transparent way, and that is not without risk."