Chlorpyrifos is an organophosphate insecticide. Pure chlorpyrifos is made up of white or colorless crystals. It has a slightly skunky odor, like rotten eggs or garlic. Chlorpyrifos is used to control many different kinds of pests, including termites, mosquitoes, and roundworms. Chlorpyrifos was first registered as an insecticide in 1965 and the United States Environmental Protection Agency (US EPA) re-registered it in 2006. The only legal indoor use for chlorpyrifos is in containers with treated baits.