The F.D.A. said it decided to take the action after years of warnings and even a visit to Chinese fish ponds that resulted in no signs of improvement. But Dr. David Acheson, the F.D.A.’s assistant commissioner for food protection, stressed that the seafood posed no immediate health threat, though long-term consumption could result in health problems.
“There’s been a continued pattern of violation with no signs of abatement,” Dr. Acheson said.
The seafood announcement comes after a string of reports in recent months about Chinese imports that have failed to meet American health and safety standards: pet food ingredients, toothpaste, toy trains and tires.
The seafood move, however, may have the broadest impact on China, the world’s biggest producer of farm-raised fish. The country is also the biggest foreign supplier of seafood to the United States, accounting for 22 percent of the total imports.
The seafood named in the F.D.A.’s “import alert” are shrimp; catfish; eel; basa, which are similar to catfish; and dace, similar to carp. Some of the contaminants cited have been found to cause cancer in laboratory animals, while others may increase antibiotic resistance. Under the import alert, the seafood can be sold in the United States only if importers provide independent testing that shows the seafood does not contain the contaminants.
Officials at the Chinese Embassy in Washington did not respond to messages seeking comment.
The announcement fueled concerns about both the integrity of Chinese products and the effectiveness of the American system for identifying contaminated food.
“The list continues to grow of Chinese imports that are dangerous to American consumers,” said Senator Richard J. Durbin, Democrat of Illinois. “There reaches a point where I think it’s clear, if China wants to live in the 21st century, then they have to produce to those standards.”
After the F.D.A. announcement, Mr. Durbin and Representative Rosa L. DeLauro, Democrat of Connecticut, called on federal officials to establish a food safety agreement with China.
Ms. DeLauro, a frequent critic of the F.D.A.’s oversight of food safety, also questioned why the agency waited so long to act.
The banned substances, primarily antifungals and antibacterials, have been used by some Chinese farmers to prevent disease among their seafood. Because they are often crowded into ponds, farmed fish and shrimp can become sick as the quality of the water becomes polluted by waste and feed.
“You may have 10 to 20 times the density of fish as in a natural environment,” said Robert P. Romaire, professor of aquaculture at Louisiana State University.
American regulators allow the use of a limited number of antibiotics. But Mr. Romaire said some of the Chinese farmers used antibiotics indiscriminately.
None of the antibiotics and food additives found in the Chinese seafood — nitrofuran, malachite green, gentian violet and fluoroquinolones — are on the approved list of regulators. Long-term exposure to nitrofuran, malachite green and gentian violet, which are also illegal in China, has been shown to cause cancer in laboratory animals.
Fluoroquinolones are allowed in Chinese aquaculture. Nevertheless, they are not permitted in fish in the United States because their use may increase antibiotic resistance for people.
The problems with contaminated Chinese seafood imports date back at least six years. Before this week, the F.D.A. had issued other, more narrow warnings about contaminated Chinese seafood beginning in 2001.
In the fall of 2006, F.D.A. officials went to China to inspect aquaculture operations and found “the residue control program ineffective.” The agency increased its inspections of Chinese seafood, starting last October, and, officials said, found that 15 percent of the samples were contaminated.
China’s seafood shipments to the United States were valued at $1.9 billion in 2006, a 193 percent increase over 2001, according to the Department of Agriculture. The biggest American imports from China are shrimp, tilapia, scallops, cod and pollock, federal statistics show, although only shrimp was affected by yesterday’s announcement.
Several Southern states, which have their own catfish and shrimp-farming operations, have already blocked the sale of some Chinese seafood. Their rules say that the seafood can be sold only if it passes testing that proves it has no contaminants.
The state of Alabama announced its ban after testing found 14 of 20 samples contained fluoroquinolones. Mississippi officials found that 18 of 26 samples of Chinese catfish were contaminated with fluoroquinolones.
“We are saying all Chinese seafood that comes in here has to be tested prior to sale,” said Bob Odom, Louisiana’s agriculture and forestry commissioner. “The simple reason for that is we found a lot of it that is contaminated.”
The F.D.A. maintains a database of imported products that are prevented from entering the United States because they do not comply with American standards. In May, for instance, the agency turned away 165 shipments from China, 49 of them seafood.
Monkfish was rejected for being filthy and unfit to be eaten, the records show. Frozen catfish nuggets were turned away because they contained animal drugs. Tilapia fillets were contaminated with salmonella.
The problems were even worse in April, when 257 shipments from China were rejected, including 68 of seafood. Frozen eel contained pesticides, frozen channel catfish had salmonella and frozen yellowfin steaks were filthy, the records show.
In a report on the F.D.A.’s oversight released in May, Food and Water Watch, a Washington-based nonprofit group, found that more than 60 percent of the seafood that was rejected at the border by the F.D.A. came from China.
The group’s report also found that the percentage of seafood shipments that were pulled out for laboratory analysis declined in recent years, from 0.88 percent in 2003 to 0.59 percent in 2006. Over all, about 2 percent of seafood imported from 2003 to 2006 received either a sensory examination for color and smell or a more detailed laboratory analysis.
Of the seafood that was refused at the border, filth was the top listed reason and salmonella was second, with shrimp accounting for about half of those cases, the report found.
Of the shipments rejected for animal drug residues in 2006, 63 percent were from China, the report found. Vietnam ranked second in rejections for animal drug residue, 11 percent.
F.D.A. officials said yesterday, however, that the agency inspected a higher percentage of Chinese seafood imports — 5 percent — because of continuing concerns about farm-raised fish from that country.