GENEVA - Some are smokers. Some drink too much. Some admit they love red meat. But virtually all shoppers here at the Migros Supermarket on the bustling Rue des Paquis are united in avoiding a risk they regard as unacceptable: genetically modified food.

That is easy to do here in Switzerland, as in the rest of Europe, where food containing such ingredients must be labeled by law. Many large retailers, like Migros, have essentially stopped stocking the products, regarding them as bad for public image.

"I try not to eat any of it and always read the boxes," said Marco Feline, 32, an artist in jeans, getting onto his bike (with no helmet). "It scares me because we don't know what the long-term effects will be - on people or the environment."

The majority of corn and soy in the United States is now grown from genetically modified seeds, altered to increase their resistance to pests or reduce their need for water, for example. In the past decade, Americans have happily - if unknowingly - gobbled down hundreds of millions of servings of genetically modified foods. The U.S. Food and Drug Administration says there have been no adverse effects, and there is no specific labeling.

But in Europe - where food is high culture, if not religion - farmers, consumers, chefs and environmental groups have joined voices loudly and stubbornly to oppose bioengineered foods, effectively blocking their arrival at the farms and on the tables of the Continent. And that, in turn, has created a huge ripple effect on trade and politics, from North America to Africa.

The United States, Canada and Argentina have filed a complaint that is pending before the World Trade Organization, contending that European laws and procedures that discriminate against genetically modified products are irrational and unscientific, and so constitute an unfair trade barrier.

U.S. companies like Monsanto, which invested heavily in the technology, suffered huge losses when Europe balked. As part of a public relations effort, the U.S. State Department enlisted a Vatican academy last month as a co-sponsor of a conference in Rome, "Feeding a Hungry World: The Moral Imperative of Biotechnology."

In response to such pressure, the European Union has relaxed legal restrictions on genetically modified foods.

In May the EU approved for sale a genetically modified sweet corn, lifting a five-year moratorium on new imports. Last month the European Commission gave its seal of approval to 17 types of genetically modified corn seed for farming. But no one expects a wide-open market.

"We have no illusion that the market will change anytime soon," said Markus Payer, spokesman for Syngenta, the Swiss agribusiness company whose BT-11 corn got the approval in May. "That will only be created by consumer acceptance in Europe."

"There is currently no inclination among European consumers to buy these things," Payer went on. "But the atmosphere of rejection is not based on facts. That is a political, cultural and media-driven decision. And so we are convinced that more and more consumers will see the benefits."

Indeed, the battle lines between countries for and against genetically modified foods seem to be hardening. Several African countries, following Europe's lead, have rejected donations of genetically engineered food and seeds. In Asia, reluctance appears to be spreading. While countries like China and India are enthusiastically planting biotech crops like cotton, genetically modified food crops are having trouble winning approval.

Africa's rejection is based partly on health and local environmental concerns, but also on economic interests: Zambia and Mozambique have discovered a good market in selling unmodified grain and soy to Europe, supplanting the United States as European suppliers.

Mauro Albrizio, vice president of the European Environmental Bureau, a policy group based in Brussels, said: "In the U.S., genetically modified foods were a fait accompli; here in Europe we succeeded in preventing that."

Genetically modified foods arrived on America's dinner plates with little fanfare in the mid-1990s as large-scale farmers in the United States enthusiastically started planting the seeds, which increased production and reduced the amount of pesticide required. Convinced that bioengineered food was "at least as safe as conventional food," the U.S. Food and Drug Administration declared that a bioengineered lemon was the same as an ordinary lemon, and did not require special labeling or regulation.

Today, nearly two-thirds of the genetically modified crops in the world are grown in the United States, mostly corn and soybeans. "In the U.S., a large part of the diet is actually bioengineered," said Lester Crawford, acting commissioner of the Food and Drug agency.

"The first thing other nations want to know is how many illnesses or adverse reactions we've seen," he added. "But we haven't actually had any problems at all with bioengineered foods."

Vast amounts of money are at stake. Believing that genetically modified foods would quickly catch on throughout the world as they had the United States, large biotech companies like Monsanto invested billions of dollars.

Since the late 1990s the European Union has required that all food containing more than tiny amounts of genetically modified materials be labeled, and that all genetically modified products be submitted for approval before sale in Europe. No products were approved during an informal moratorium from 1998 to 2003. In the past five years, many parts of Europe have enacted local bans on growing such foods.

In fact, most scientific panels have concluded that "foods derived from the transgenic crops currently on the market are safe to eat," in the words of a recent report from the UN's Food and Agricultural Organization. But the report also cautioned that crops must be evaluated case by case.

And low risk is not no risk. The 87 member states of the UN-sponsored Cartegena Protocol on Biosafety required labeling this year of all bulk shipments of food containing genetically modified products. The United States has not signed the pact.

More important, though, is that the assessment of risk depends largely on the degree of proof that a country's consumers demand.

"In their personal lives people take lots of risk - they drive too fast and bungee-jump - but for food their acceptance of risk is very low," said Philipp Hübner of the Basel-Stadt Canton Laboratory in Switzerland, which tests products in that country for contamination with genetically modified organisms. But Hübner sees his work as detecting fraud in labeling rather than as safeguarding the public health.

"For most scientists it is not so much a safety issue, but an ethical and societal question," he said. "This is what the public here has chosen, like Muslims choosing not to eat pork."

In a survey by the European Opinion Research Group in late 2002, 88.6 percent of Europeans listed the "quality of food products" as an environmental issue with health implications.

But health fears, which can move markets, are not always consistent. In some parts of Europe, like Bordeaux, that have declared themselves free of genetically modified organisms, energy is supplied by nuclear power plants.

To sell Sugar Pops cereal to European consumers, Kellogg's imports unmodified corn from Argentina and spends extra money to make sure that the entire transportation and processing chain is free of bioengineered products, said Chris Wermann, a company spokesman. The same cereal contains genetically modified corn in the United States. Both varieties contain all the usual sugars, artificial colors and flavors.

European advocates defend their right to be finicky. "This is not ideology - it's a pragmatic stand because of potential risks to health and the environment," said Albrizio of the European Environmental Bureau, noting that there is some evidence that genetically modified crops may trigger more allergies.

In terms of agriculture, there are some very clear-cut effects, since genetically modified seeds tend to spread in the environment once they have been planted, making it hard to maintain crops that are organic and free of genetic modification. Scientists call this phenomenon "co-mixing."

To environmentalists and especially to farmers, "co-mixing" it is potentially devastating "contamination." That is why the farmers of Tuscany and 11 other regions of Italy have declared themselves free of bioengineering.

In fact, European farmers and consumers have so far created a firewall against genetically modified organisms, one that the changing laws and World Trade Organization challenges may not breach easily.

"In theory you could sell GMO products here, with labeling," Hübner said. "But I'm not aware of any products that are now being sold, because no store wants them on their shelves."