In the United States, where about 4 percent of food imports come from Japan, the Food and Drug Administration has restricted some foods from that country. The agency is working with customs officials to screen incoming fish and other food for traces of radiation. So far that screening has identified seven items that required further testing to see if the detected radiation exceeded normal background levels, according to Siobhan Delancey, an FDA spokeswoman. Those items included tea and flavoring compounds. She said three of the items had been cleared for delivery and four were awaiting test results. Patricia A. Hansen, a senior scientist at the FDA, acknowledged that the radiation detection methods used to screen food imports were not sensitive enough to detect a single contaminated fish in a large shipment, but said small amounts of contamination did not represent a public health hazard. A person would have to consume large amounts of fish in excess of what are known as an "intervention level," or threshold level, of radiation for an extended period before it would be considered dangerous. "One fish that might be at an intervention level in a huge cargo container, we're not going to pick that up," she said. “But the important context is whether one fish at the intervention level is a public health concern. No, it is not.”

Nicholas Fisher, a professor of marine sciences at the State University of New York at Stony Brook, said that, according to some radiation safety guidelines, people could safely eat 35 pounds of fish each year containing the level of cesium-137 detected in the Japanese fish. “You’re not going to die from eating it right away,” he said, “but we’re getting to levels where I would think twice about eating it.”

All the talk about radioactive food in Japan, which earlier banned milk and other farm products from areas near the crippled plant, has made some people uneasy, even thousands of miles away.

“When radioactive material started going into the ocean, that raised my concern greatly,” Karen Werner, 68, said as she shopped for fish at 99 Ranch Market in Richmond, California. “Right now, I’m not too worried about it showing up in fish, but I’m keeping my eye on it.”

Lee Nakamura, a partner who manages the fish counter at Tokyo Fish Market in Berkeley, California, estimated that one in five customers asked about possible radiation, but he had not yet seen an impact on sales. He said his Japanese suppliers had assured him that fish were being tested for possible radiation. “Everything is under a microscope right now,” Mr. Nakamura said. “I feel confident the fish is safe. Everyone in Japan and here is looking at it and double-checking it before it gets to us.”

Several restaurant owners and fish importers said that while they continued to buy some fish from Japan, it came from areas far from the reactor site.

Still, Scott Rosenberg, an owner of Sushi Yasuda, a highly regarded sushi restaurant in Manhattan, said he planned to buy a radiation detector and would post a notice on the restaurant’s website to let customers know about the testing. “We want to make sure there is no exposure,” he said.

Other segments of the food industry are also grappling with how to respond to radiation concerns. Sensitive monitoring devices and tests have detected trace amounts of radioactive material from Japan in the air and water in many states. Tests in Arizona, California and Washington State have found minuscule amounts in milk, leading to concern among dairy farmers. Everything detected has been well below levels considered dangerous, but food companies realize that consumers may still need to be reassured.

In California, Will Daniels, senior vice president for food safety at Earthbound Farm, a major producer of organic salad greens, said the company was prepared to test soil and greens for radiation if concerns persisted or fallout from Japan intensified. "The likelihood of contamination on the West Coast is extremely low, so it’s really important that we’re monitoring appropriately and not creating panic," Daniels said. "But we certainly need to make sure we’re doing the appropriate thing and are ready to respond."

Cliff Coles, a consultant who works with Earthbound and other produce companies and food-ingredient importers on food-safety issues, said he had ordered two radiation detectors and was planning to take them into fields where greens, tomatoes and peppers would be grown this spring. He said he would work with Earthbound’s growers to make sure the fish-emulsion fertilizer they use was tested for radiation. "We’re just trying to get our clients to be proactive and say that, while this may not be the end-all solution, let’s take a look at what’s going on around us before we get blindsided," Coles said. Consumer worries about radiation have led to a big boom in sales of one food that often comes from Japan: seaweed.