Recent laboratory tests found so much mercury in tuna sushi from 20 Manhattan stores and restaurants that at most of them, a regular diet of six pieces a week would exceed the levels considered acceptable by the Environmental Protection Agency.

"No one should eat a meal of tuna with mercury levels like those found in the restaurant samples more than about once every three weeks," said Dr. Michael Gochfeld, professor of environmental and occupational medicine at the Robert Wood Johnson Medical School in Piscataway, N.J.

Dr. Gochfeld analyzed the sushi for The Times with Dr. Joanna Burger, professor of life sciences at Rutgers University. He is a former chairman of the New Jersey Mercury Task Force and also treats patients with mercury poisoning.

The owner of a restaurant whose tuna sushi had particularly high mercury concentrations said he was shocked by the findings. "I’m startled by this," said the owner, Drew Nieporent, a managing partner of Nobu Next Door. "Anything that might endanger any customer of ours, we’d be inclined to take off the menu immediately and get to the bottom of it."