Consumer Reports said 6 percent of canned light tuna tested by the FDA "contained at least as much of the metal -- in some cases more than twice as much -- as the average albacore." Most of the cans had only one-third as much mercury.

Jane Halloran, director of food policy initiatives for Consumers Union, said due to concerns that both types of tuna showed instances of higher levels of mercury, it decided to recommend pregnant women eat neither. Consumers Union publishes Consumer Reports.

The FDA said its extensive testing of canned light tuna showed an average mercury content of 0.12 parts per million, one-tenth the level found in species that it advises pregnant women to avoid.

The FDA said there is no research suggesting harm from an occasional serving of tuna that has a higher than average level of mercury.

The U.S. Tuna Foundation trade group said the magazine was overreacting to a minor problem. It said the nutritional benefits of seafood easily outweigh the risk posed by "trace amounts of mercury" and said scientific research shows the federal guidelines on consumption are sound.

Two consultants to the tuna industry said the Consumer Reports advice was not consistent with the conclusions of most researchers.

Dr. Joshua Cohen of the Tufts New England Medical Center said mercury risks to unborn babies "depends not on exposure on any given day but on the average exposure over several weeks."

The three major U.S. tuna companies are Del Monte Foods, maker of StarKist tuna; Bumble Bee Seafoods, a unit of Connors Brothers Income Fund of Canada; and Tri-Union Seafoods, maker of Chicken of the Sea.

Consumer Reports said the higher levels of mercury sometimes found in canned tuna might come from yellowfin tuna, which tends to carry more mercury than skipjack, which is usually used in light tuna.