The alert means that the fish will be allowed for sale in the United States only if testing proves that it is free of those substances.
While the federal agency stopped short of an outright ban, the alert is nonetheless hugely significant because China is a major source of imported seafood in the United States, accounting for 21 percent of total imports.
The United States imports 81 percent of the seafood that is consumed here.
“There’s been a continued pattern of violation with no signs of abatement,” said David Acheson, the F.D.A.’s assistant commissioner for food safety. He insisted that there was no imminent danger to human health, but that prolonged consumption could cause health problems.
The other varieties affected by the ban are eel, basa, which is related to catfish, and dace, which is related to carp.
The seafood alert is the latest and perhaps broadest indictment yet against Chinese products, which have come under increasing scrutiny in recent months after pet food, toothpaste, toy trains and tires have been found to be contaminated or defective in some way.
China is the world’s leading producer of farm-raised fish. Its shipments to the United States were valued at $1.9 billion in 2006, a 193 percent increase from 2001, according to the Department of Agriculture. The biggest American imports from China are shrimp, tilapia, scallops, cod and pollock, federal statistics show.
The move by the F.D.A. comes after several Southern states have already blocked the sale of Chinese seafood contaminants. Now, Chinese catfish 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, a type of antibiotic banned by the F.D.A. 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 problems with Chinese seafood are evident in a database of products that the FDA stops at the border. In May, for instance, the F.D.A. turned away 165 shipments from China, 49 of which were seafood.
Monkfish was rejected for being filthy. Frozen catfish nuggets were turned away because they contained veterinary 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 organization, found that more than 60 percent of the seafood that was rejected at the border by the F.D.A. came from China.
The report also found that the percentage of seafood shipments that were pulled out for laboratory analysis declined in recent years, from .88 percent in 2003 to .59 percent in 2006, the report found. Over all, about 2 percent of seafood import shipments between 2003 and 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 reason and salmonella was second, with shrimp accounting for about half of those, the report found.
Of the shipments rejected for veterinary drug residues in 2006, 63 percent were from China, the report found. Vietnam had the second most rejections for veterinary drug residue, 11 percent.
The Government Accountability Office has also criticized the F.D.A.’s oversight of seafood imports. In a 2004 report, the G.A.O. determined that the seafood inspection program had improved from 2001, when the agency concluded that the seafood inspection program did not sufficiently protect consumers.
But the G.A.O. also found that the F.D.A. still had considerable room for improvement.