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.

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.