Dominican immigrants in Washington Heights have long had a ready weapon against vermin: Tres Pasitos, or Three Little Steps, a granular product so named because after rats eat it, they walk three steps and die. The heavily regulated insecticide Tempo used to be peddled openly in Washington Heights. But for all its effectiveness, Tres Pasitos is also illegal. Manufactured in the Dominican Republic, it is imported and sold on the black market in unlabeled containers or small plastic bags like those used by drug dealers. “It’s not registered, not controlled, there’s no antidote,” warned Andy Linares, president of Bug Off Pest Control Center in Washington Heights. “It’s really pretty scary.” The recent arrests of vendors in Chinatown accused of selling an illegal, highly potent pesticide apparently smuggled from China opened a window on the underground trade in chemicals used to combat roaches, rats and other vermin in the city’s immigrant neighborhoods. The product at the center of that investigation, which government officials disclosed this week, was sold in a box printed with Chinese characters and an awkward English translation: “The Cat Be Unemployed.” Officials said the substance contained a toxic chemical in a concentration almost 61 times higher than federal regulations allowed. Though several professional exterminators said they had never encountered the product, they said it was just the latest in a stream of goods that for years had flowed from overseas factories into the supply closets of households and businesses in New York. City health officials said that since 2005 they had received about 100 reports of poisoning from Tres Pasitos and two other anti-pest products favored by immigrants, Tempo and Chinese Chalk. While there were no deaths, about 40 percent of the victims had to be treated in medical centers, officials said. Lower prices and language barriers can steer immigrants toward the underground market, immigrant advocates say. So can cultural biases: immigrants become familiar with products in their homeland and import their preferences, and sometimes the substance itself, when they move to the United States. They pass along their knowledge and techniques to neighbors and friends. “It’s word of mouth: get this and do that,” said Gil Bloom, owner of Standard Pest Management, a company founded in 1929 and based in Long Island City, Queens. Government officials have tried for years to clamp down on the trade in Tres Pasitos, which is often used against rodents and which contains aldicarb, an insecticide that is highly toxic to humans and pets. Yet demand remains high. “The people will do what they want to do,” said Omar Campos, co-owner of Exterminadora Latina, a company in Rego Park, Queens. “They don’t care if it murders a kid.” Yaneyri Caney, a Dominican immigrant who works at a cafe on Broadway in Washington Heights, said that while she did not use Tres Pasitos because she did not have a rodent problem, she was a devoted user of Tempo. Sold as a powder, Tempo is legally available in New York only to licensed applicators. But for years it has been traded briskly on the black market in small quantities. It is supposed to be mixed with water at ratios provided by the manufacturer, but untrained users often sprinkle it undiluted around their homes at hundreds of times its recommended concentration. Ms. Caney said she bought it in packets from street vendors and, every couple of weeks, poured the powder in a line around the base of her bed and spread it on her kitchen cabinets and walls to deal with cockroaches. “It’s far more effective” than products like Raid and Combat, she said. Ms. Caney said she was aware of the danger. “If there are children around,” she cautioned, “you have to watch out.” Residents and shop owners in neighborhoods like Washington Heights and the South Bronx said that in the 1990s, vendors would openly hawk Tempo from folding tables on the sidewalks. “They were going: ‘Tempo! Get your Tempo!’ ” recalled Mr. Linares, the exterminator in Washington Heights. “It was out of control.” Since then, a clampdown by the authorities has driven the market further underground. In bodegas throughout Washington Heights, store owners who were asked this week for Tres Pasitos and Tempo reacted as if the request were for crack cocaine. “No way,” one bodega owner said, his eyes widening in alarm. “That’s illegal. You’ll have to get it on the street.” Exterminators said the liveliest sector of the underground pesticide market was the trade in Chinese products, with new substances regularly entering the stream of shadowy commerce. For years, one of the most popular black market products from China was a stick of chalk loaded with insecticides, including the chemical deltamethrin. Known as Chinese Chalk or Miraculous Chalk, it was used to draw a line along a wall or around a piece of furniture, like a bed or a kitchen table. Insects would die when they touched the line. The arrests of several vendors in the city, and a public education campaign warning of the chalk’s dangers, apparently chilled the market. Professional exterminators say customers no longer request the product as often as they used to. Still, exterminators tell horror stories of do-it-yourself efforts gone awry. One practice — used by some immigrants, experts said — involves dousing furniture in kerosene or gasoline to combat bedbugs and other pests. In their homeland, the immigrants would have hauled furniture to the yard. But in New York, with no yard available, some families have left their furniture where it stood and sprayed the flammable liquids inside. In some cases, firefighters have been forced to respond.