"WHEN IS a crab . . . Not a crab?" says the promotional brochure from Jac Creative Foods. When it is pollack, we answer. Or cod, or croaker. And when those fish are deboned, pressed, mixed with crab, crab extract or crab paste, extruded by machines, formed into batons and topped with artificial food coloring.

In Washington, this type of product has been labeled at some supermarket deli counters and carryouts as crab meat. "If it's called 'crab meat,' it has to be 100 percent crab meat," said Herbert Wood, deputy chief of the district's Bureau of Environmental Health. Otherwise, he said, "it is indeed a violation of the truth-in-menu [regulation]," which applies to restaurant and carryout food and to supermarket food that has been re-packaged in the District of Columbia.

At the Georgetown Safeway on Jan. 21, the deli department sign for the processed sea legs read "Crab Meat." The price: a hefty $7.99 a pound. Asked what the product was, the deli clerk replied, "Alaskan king crab legs." But, she added, "I am supposed to tell you it's not 100 percent crab." (Giant sells another brand. Ocean Magic Seafood pieces, in an eight-ounce frozen-food box for $1.69.)

On Wednesday, the Safeway at 1525 Wilson Blvd. in Arlington was selling "Crab Salad" for $7.95 in its deli department, using the same product. This time the deli clerk said the salad was made entirely from Alaskan king crab leg.

Safeway spokesman Ernie Moore said the product, Sea Leg Blend, is carried in 12 area Safeway stores. The labeling at the Georgetown and Arlington stores, he said, was erroneous. Moore said he sent a bulletin to all stores after learning of the problem, reiterating that the correct label is "Sea Leg Blend" and that "it must never be called Alaskan king crab legs."

The Dutch Treat carryout at 1710 L St. NW has been selling a "Crabmeat Salad" for $3.55 that is, according to manager Bill Verhoeven, made with a product called Sea Legs, which is 35 percent crab meat. "It looks like it crab meat and tastes like it," said Verhoeven. Besides, he said, a salad of pure crab meat would cost customers about $7.

It's not hard to understand the temptation to market an imitation. Real Alaskan king crab is averaging about $13 a pound in Washington supermarkets and Alaskan fishery sources report the crustacean has been overfished in the midst of what they term its "seven-year down cycle," while species such as pollack, croaker and cod are plentiful and underused. The "American public would accept this product because they accept things like hot dogs," said Diane Boratyn of the National Marine Fisheries Service. But, said Boratyn, "the way to market it is to tell everybody what it is."

Sold to wholesalers and retailers under brand names such as "Seafood Sections," "Sea Legs" and "Ocean Pieces," the product has its roots in Japan. The process and the fish mixture were developed by manufacturers there. The base of the simulated crab legs is sirimi, a fish paste used by generations of Japanese in making fish cakes or konobako. A finished simulated crab leg averages about 60 to 80 percent sirimi (processed pollack, frequently caught in the Bering Sea, or croaker or cod) and could have from 0 to 40 percent crab, plus flavoring agents, binder, seasonings and food coloring. The ingredient label for King Krab Simulated Crab Legs, for instance, reads: "Fish Meat (Cod and Croaker), water, egg white, modified food starch, sugar, salt, crab paste, monosodium glutamate, glycine, crab flavor, tripolyphosphate, U.S. certified natural and artificial food coloring."

The imitation is sold wholesale in uniform sticks, bite-size pieces or shreds. Frank Kawana, president of Jac Creative Foods, manufacturers of King Krab Simulated Crab Legs, said the sirimi mixture is shredded and rolled, "like a cigarette." Typing-paper white, with a bright reddish-orange surface, the uniformly cylindrical "legs" contain strands of extruded fish mixture that are evident at the cut ends. The spaghetti-like strands of the seafood legs can be pulled apart.

Japanese-manufactured simulated crab legs have been available in this country for over five years, but have become considerably more visible locally during the past year or so. Only one company, Jac Creative Foods, manufactures the sirimi base and simulated crab legs in the United States--in Bayou Labatre, Ala. It uses croaker caught by gulf shrimpers, who previously had thrown much of it overboard.

The rest of the simulated crab legs are imported frozen to the United States, according to Al Nigorizawa, director of sales for Fishking Processors, a Los Angeles company that sells Mrs. Friday's Seafood Sections. Last year, Japan exported 12 million pounds to more than a half dozen domestic brokerage companies and marketing arms of Japanese companies.

"From a marketing standpoint, 'crab' rings bells," said Nigorizawa. It can also ring alarms. Last month, the FDA, through the Bureau of Customs, refused to permit The Kroger Company to import "Crab Shapes." The FDA and the company then reached a compromise. "Crab Shapes" could come into the country if placards were placed in stores indicating that the fish mixture was "imitation crab." In addition, Kroger agreed to change the labels on future shipments.

According to The Kroger Company's attorney, the original label was proper. He pointed out that underneath the name "Crab Shapes" was written "40 percent crab," thus consumers would not have been deceived.

Although not required to seek FDA approval of food labels before domestic products go on sale, several manufacturers have written to FDA asking its advice on proposed labels that called the imitation product "crab." According to Eugene Newberry, chief of the Case and Advisory Branch in the FDA's Bureau of Foods, for the most part, they have followed FDA's suggestions.

According to Newberry, once products enter interstate commerce, FDA can insist that food labels contain an appropriate statement of identity.

Whether or not the labels are misleading, the products are "nutritionally inferior" to crab, said Newberry. Imitation crab legs contain at least 10 percent less protein than pure crab, he said. Newberry said he fears further misunderstandings even with correct labeling. Pollack is unfamiliar to many Americans, he said, and "consumers may not understand it's a fish."

That's not the only thing consumers may not understand about simulated crab legs. The other question for American tastes is how to eat them. According to Fishking's Nigorizawa, the product was developed for the new, Americanized generation of young Japanese who "are not finding the taste of fish cakes acceptable." Although the export market is the target, the simulated crab legs are more easily understood in Japan, said Terry Kishimoto, general manager of Jac Creative Foods, the company that manufactures King Krab. "Japanese people are more aware of the uses of fish," she said. "The American people just know fillets."

So the companies' promotional material gives recipe ideas for the simulated seafood in dips, soups and salads that include ingredients Americans can understand, said Kawana. "In Japan, they would never come out with recipes that include cream cheese."

Taste opinions vary on the simulated crab legs. Verhoeven said 80 percent of his customers at Dutch Treat like the salad. Informal taste tests of various brands held at The Washington Post drew comments ranging from "watery and rubbery," "too sweet" and "weird texture" to "not bad" and "tastes like crab meat."

Local sales agents and distributors of the product say they're finding it better suited for the institutional than the retail marketplace. Giant spokeswoman Sue Portney said the boxed "legs" are available to all stores, but few have been ordering them. They "haven't sold well," said Portney.

Versatility is the product's selling point to institutions. Bob Hartpence, Washington metro area sales supervisor of Smelkinson Brothers, a distributor in Laurel, said it has many different market areas: as a hors d'oeuvre for happy hour ("people think they're eating king crab"), as an ingredient in a seafood newburg for fast food sit-down restaurants, as a seafood salad in delicatessens.

At Yummy Yogurt, the "seafood salad," a garden salad with shrimp and strips of imitation crab legs that sells for $3.75, is "a heck of a mover," said Sheldon Fisher, owner of the take-out chain. He spotted the product at a food show and was attracted by its cost compared with crab meat.

The product doesn't sell at white-tablecloth restaurants, which can afford to serve the real thing, local distributors say. Biggest sales are in the institutional marketplace--such as airlines and hospitals--where the aim is a less expensive seafood extender.

If manufacturers get their way, simulated crab legs are only the first in a line of possible hybrid seafood. Jac Creative Foods has developed a simulated scallop and reports that simulated abalone, shrimp and lobster are on the way.