WASHINGTON – In March, inspectors checking Chinese seafood arriving at U.S. ports made some unsettling discoveries: fish infected with salmonella in Baltimore and Seattle and shrimp with banned veterinary drugs in Florida.

And a shipment intercepted in Los Angeles labeled "channel catfish" wasn't catfish at all.

"A lot of those products coming in from overseas, you have no clue as to what is in them," said Paul Hitchens, an aquaculture specialist in southern Illinois, where cut-rate Chinese catfish threaten the livelihood of fish farmers.

China has rapidly become the leading exporter of seafood to the United States, flooding supermarkets and restaurants.

And while China agreed late last year to improve the safety of its food exports, the inspectors' March findings were not isolated cases.

According to Food and Drug Administration records, inspectors turned away nearly 400 shipments of tainted seafood in a year's time from China.

The records told a troubling tale, but more troubling was what they didn't tell. Only a tiny fraction of imports are inspected, and even fewer are tested.

It's a challenge the United States is just starting to confront: In an increasingly globalized food supply, the government is unprepared to keep Americans safe.

"When you look at less than 1 percent of shipments, and sample and test maybe one-fifth of those, there's no way you can protect the American food supply," said Michael Taylor, a former FDA official who is professor of health policy at George Washington University.

Seafood is considered one of the riskier imports, and those from China have risen steadily. When the FDA does turn away shipments, usually it is because they contain veterinary drugs.

More than 100 of the shipments were rejected for being filthy, decomposed or otherwise unfit for consumption, according to the records.

In December, after disclosures about Chinese imports of poisonous pet food and lead-filled toys, the FDA and the Chinese government agreed on new procedures aimed at preventing tainted and dangerous food and drugs from reaching the U.S. But skeptics question the voluntary arrangement.

Meanwhile, Chinese seafood is a prime target of legislation in Congress to revamp decades-old inspection mechanisms.

FDA officials are requesting new authority, including the ability to license private companies to assist with inspections. But the Bush administration has signaled opposition to key provisions that would require regular inspections in foreign lands and limit ports where food can arrive to docks with FDA labs.

William Hubbard, formerly the FDA's associate commissioner, said that the FDA's inspection system was designed early last century, when the big challenge was finding bugs or mold in barrels of commodities such as flour or molasses. Now, the U.S. gets millions of shipments each year from around the world.

"Increasingly, the world is moving in a better direction in food safety, and we're falling behind. As our system becomes more antiquated and more ineffective, the world is sending us their junk," Mr. Hubbard said.