SEATTLE (AP) _ Hundreds of companies selling everything from bagels to chocolate flowers to snails opened shop Wednesday at a fair organized to whet the appetites of foreign buyers - especially the Japanese - for American food.

Organizers of the National Food and Agriculture Exposition, sponsored by the U.S. Department of Agriculture and state agriculture departments, say the three-day fair has attracted 1,500 foreign buyers and representatives of more than 350 U.S. companies.

Tables inside the Seattle Coliseum tempted fair-goers with samples of American wine, jellies, salmon, bagels, snails and chocolates.

Electric frying pans turned out samples of marinated meat, beer salesmen poured cups of brew, and condiment producers discussed how their products could be made spicier or milder, depending upon national taste.

Japan has been targeted by the food companies because the dollar is relatively cheap against the yen, and Japanese consumers prove ravenous for American food.

Earlier Wednesday, Agriculture Secretary Richard Lyng, who opened the conference, said he was disappointed in recent trade talks in Japan because there was little movement by the Japanese to drop trade quotas of such commodities as beef and citrus fruits.

The Japanese External Trade Organization is making the fair the start of a 17-day mission to the United States to encourage the export of processed food products to Japan and to explain exactly what the Japanese buyer wants.

The task is an exact science, says William Smiley Jr., Washington state's trade representative in Japan who is promoting things like the state's asparagus and apples.

The Japanese are ''the most particular consumers you'll find anywhere in the world,'' he said. ''They will demand quality and are willing to pay a higher price.''

This month, Washington growers shipped about 100 tons of asparagus to Japanese supermarkets, all delicately calculated to appeal to the Japanese buyer.

Each spear was 9 inches long, between 8-16ths and 9-16ths of an inch in diameter, and sold in five-stalk bundles.

Elias Demotriades, president of Demco of Brookline, Mass., brought eggs and live lobsters.

Rex Reed of Abuquerque's Chocolate Roses Inc. said some Asian importers were ''very, very interested'' in his candy flowers, which he said are equally good in desserts or floral displays.

Judith Shepherd of Maury Island Farms Co. said she doesn't know why, but Asian buyers especially like her four-jar gift packages of jams and jellies, while Canadians like the preserves in a single larger jar. For that matter, she said, Californians like jelly a little tart, while Northwesterners don't.

D.B. Wilcox, here to sell the Allegro meat marinade he makes in Paris, Tenn, said one purpose of such trade shows was ''to see if there's an interest in a product's taste from a given nationality.''

For his product, he has found ''the Japanese like it on the mild side, while the Arabs like it the other way around.''

Competition in Japan is intense, with American producers facing Europeans, Australians, Koreans, New Zealanders and others, said Motoharu Hirakawa, head of the JETRO mission and manager of Marubeni Corp.'s Food Development Division.

Hirakawa said he divides Japanese consumers into two groups, those who can remember the hunger after World War II, and their children, who grew up with Western-style food beginning with U.S. aid programs.

''Until recently, it has been enough for food just to be delicious and inexpensive,'' Hirakawa said through a translator.

''But nowadays people will choose a restaurant on a different basis, because it's fashionable, service is good or because they like the waitresses or waiters.''

Takayoshi Mori, JETRO's director, said Japanese, have taken strongly to American food. The trend is particularly obvious among young people, who frequent MacDonald's and other fast food outlets, he said.