To many food lovers, sushi has become a near religion, and a cornerstone of the faith is that the fish is extraordinarily fresh. Its priests are chefs with seemingly mystical abilities to summon fresh fish from all corners of the globe.

But because of health concerns and growing demand, 50 to 60 percent of sushi in the United States is frozen at some point in its journey from the ocean, according to wholesalers. And rare is the sushi restaurant that tells customers upfront that they may be eating fish that has been in deep freeze for up to two years.

Most would be even more surprised to learn that if the sushi has not been frozen, it is illegal to serve it in the United States.

Food and Drug Administration regulations stipulate that fish to be eaten raw -- whether as sushi, sashimi, seviche, or tartare -- must be frozen first, to kill parasites. ''I would desperately hope that all the sushi we eat is frozen,'' said George Hoskin, a director of the agency's Office of Seafood. Tuna, a deep-sea fish with exceptionally clean flesh, is the only exception to the rule.