William Grayson was strolling shoulder to shoulder with his wife, Helen, through the deli section of the McLean Giant last week when he spotted something he found extraordinary.

"Look! Sushi in a grocery store!" he said, thumbing a plastic box in the refrigerator case. "And they've got the wasabi and the gari," Helen Grayson said approvingly while turning the little tubs of the sushi condiments -- a green horseradish-like paste and shaved pickled ginger strips -- this way, then that.

The Graysons, retired foreign service officers in their early sixties, had been stationed in the Far East. They know all about this Japanese specialty made with seasoned rice, vegetables and seafood. But William Grayson never expected to find sushi in a suburban supermarket.

Would they buy it? "Sure, we'll give it a try. It looks attractive," Helen Grayson said.

Raw fish atop a wad of rice has gone really mainstream. Squirm if you must. Or, savor the moment. Sushi is now at a grocery store near you. Who's the target audience?

The Graysons are not typical of their generation. It's the younger crowd, for the most part, that's grabbing ready-to-go containers of nigiri (raw or cooked slices of seafood atop ovals of seasoned rice) as well as California rolls (pressed cylinders of imitation crab, avocado, seaweed and rice, often coated with tiny dots of fish roe or sesame seeds).

Sushi doesn't appeal to everyone. Still, Giant is giving it a go. "It's a yuppie thing, a superspecialty item," says John May, Giant Food's director of seafood purchasing.

In fact, sushi is so special that it's one of the few things supermarkets contract out to outside vendors. Why? "It's not an easy thing to make," says May. "This way we don't have to look for sushi chefs." But supermarkets don't have to look far. There are several companies that bring the chefs to them.

Over the past three months, ACE Express Sushi, a California-based corporation that operates 250 outlets in supermarkets from Seattle to Tampa, has set up sushi stations, in one form or another, in 22 Giant stores. That number will increase to 50 by this summer. And Giant isn't the only one in the sushi-to-go business.

Beyond the maze of tires and pallets piled with electronic equipment at three local Costco warehouse stores, there are combination sushi trays and party platters. Safeway has ACE Express bars in many of its West Coast stores; as for here, spokesman Gregory Teneyck says, "We're contemplating it." And specialty grocer Sutton Place Gourmet has done nigiri for more than two years.

Long available at local Japanese restaurants, sushi first appeared on the Washington-area supermarket scene at the Rockville Fresh Fields store in the spring of 1995 -- nearly a decade after sushi-to-go hit stores in California. Sushi "was an experiment," says Fresh Fields Whole Foods spokesman Joe Dobrow. "But it was so immediately successful that we had a rapid rollout into our other stores," he says.

And where did Fresh Fields find the chefs to make the sushi? It signed a contract with Advanced Fresh Concepts (AFC), another California-based company, which has installed 350 in-store sushi bars country-wide since 1986. Last year AFC racked up sushi sales of $30 million. AFC operates sushi bars in a manner similar to that of Giant's partner, ACE.

Here's how it works. The supermarkets supply the work space and storage. AFC staffs the store with a trained sushi chef (AFC chefs receive one month of training, ACE chefs receive up to 6 months of on-the-job training) as well as the raw materials, condiments and packaging. The rice arrives at the store pre-cooked and the fish pre-sliced and frozen.

Fresh Fields ended its contract with AFC after one year. Why? Highly rated Sushi-Ko, Washington's first sushi bar and restaurant, located in Glover Park in the District, offered to set-up sushi stations in nine Fresh Fields stores.

Sutton Place Gourmet considered both AFC and ACE, but decided instead to go with Taki Gourmet, a Virginia-based management company that operates sushi bars in New York and Connecticut.

"They had the best product and do a beautiful job," says Steven Roberts, vice president of Sutton's culinary operations. The fresh seafood is selected each day by longtime Sutton seafood buyer Jeff Groelig from the Jessup, Md., fish market. Taki supplies Sutton with experienced chefs.

Costco has its own way of doing things. At the Pentagon City location, Chanthaboun Sayaraph, a Laotian-born sushi chef who was trained at a Rosslyn sushi bar, feeds precooked rice into the hopper of an automatic sushi-roll machine. At the press of a button, a flattened six-by-eight-inch mat of gummy rice moves out onto a little platform, ready for the fillings.

Chanthaboun layers on seaweed, some avocado, some canned Dungeness crab and pushes another button. Bam! The platform swallows the whole business and out pops a rather sloppy "California Wrap." The entire process, which takes one minute per roll, brings sushi to the masses.

But some well-known sushi chefs don't welcome machine-made sushi or the supermarket sushi explosion.

"I have a very big concern about it," says former Sushi-Ko executive chef Kaz Okochi, who is currently developing recipes for the trendy Raku restaurants at Dupont Circle and in Bethesda. "Five years ago you had to go to a reliable Japanese restaurant for sushi. Now it's not that special anymore. There are all these sushi chefs making sushi {who} don't know anything."

Masuo Kawasaki, chef and owner of Kawasaki, a downtown sushi bar frequented by many Japanese businessmen, calls supermarket sushi "an insult to the tradition of Japanese cuisine." Says Kawasaki: "Sushi is something alive. And now people who know nothing about it are doing it like sandwiches. They miss what is fundamental -- to see the freshest of the season in front of your eyes and eat it immediately."

But Kawasaki admits that what this trend boils down to is "a matter of taste and finance." Traditional sushi bars are a luxury for most people. It's easy to run up a $60-per-person tab.

On the other hand, the buying power of the nationwide vendors allows them to offer sushi for a fraction of the price. ACE Sushi Express sells 250,000 pieces of sushi every day. Says Harlan Chin, ACE's vice president of sales and marketing: "There's plenty of business out there for everyone." Buying the Best Sushi

Sushi is best when it's freshly made. Every minute that this highly perishable food sits around, the rice is drying out and the seafood is losing flavor and freshness. Sushi shoppers should check the expiration date even though it is the policy of the stores to replace all products every day. And, it's not a bad idea to ask the deli staff or sushi chef exactly when the sushi was made that day. (In Japan, packaged sushi is stamped with the exact hour and minute it was made.) Better yet, ask the chef, if possible, to make a new order.

Here are the stores where we found sushi. Each chain uses a different supplier, so the quality varies tremendously. Order party trays in advance and request that they be prepared just before you're ready to pick them up. COSTCO Ready-to-go packs of sushi range in price from $6.49 for the California Wrap -- three large, machine-made rolls filled with Dungeness crab and avocado, which can be cut into 24 pieces -- to $12.99 for the Pacific Rim -- a combination pack with 11 pieces of tuna, shrimp, salmon and inari (tofu skin) as well as three California Wrap rolls. A party platter with 56 assorted pieces is $21.99. Sushi makers are available to make party trays but do not make custom orders. Available on a trial basis at Costco warehouse stores in Fairfax, Pentagon City and Gaithersburg. FRESH FIELDS WHOLE FOODS Chefs trained by Sushi-Ko, Washington's first sushi bar, create fresh sushi daily that ranges in price from $2 for a six-piece carrot roll to $8.99 for the seven-piece Nigiri Deluxe, which includes tuna, shrimp, salmon, eel, clam and inari. A 40-piece custom party platter is $24.99. The Arlington store makes the most unusual custom orders. Chefs are normally on duty from 10 a.m. to 7 p.m. Available at nine Fresh Fields Whole Foods stores. The Springfield store does not carry sushi. GIANT FOOD The Ace Express Fresh Sushi counter has ready-to-eat trays priced from $3.29 for a 12-piece cucumber roll to $5.99 for a 16-piece Marina Special Deluxe tray, which includes imitation crab/avocado, tuna and smelt roe rolls. The 50-piece ACE House party platter is $29.95. Most of the 22 Giant stores that offer sushi have a chef on duty from 8 a.m. to 6 p.m. for special orders and party platters. SUTTON PLACE GOURMET Grab-and-go containers range in price from $3.95 for an eight-piece cucumber roll to $7.95 for the 10-piece Nigiri Sushi Combo -- four pieces of fish sushi and six pieces of imitation crab/avocado roll. A custom-made party platter with 52 pieces is $59.95. Chefs are normally on duty from 10 a.m. to 7:30 p.m. Available at all six area Sutton Place Gourmet stores as well as Sutton on the Run, 1647 20th St. NW; call 202-588-9876. Make Your Own Sushi AFC, the largest supplier of sushi to supermarkets nationwide, has created the Ultimate Sushi Kit. It has all you need to make sushi (except the fresh fish). This attractive and reasonably priced collection of sushi-related products and a how-to book, a great guy-gift, is $34.50 (shipping extra). Call 310-604-3200 or visit the AFC Web site at afcsushi.com. CAPTION: Above, assorted sushi; at right, a combination tray, available from Costco warehouse stores in Fairfax, Pentagon City and Gaithersburg.