Terbufos (S-tert-butylthiomethyl O,O-diethyl phosphorodithioate), a systemic insecticide, is the fourth most commonly used organophosphate insecticide (OP) in the United States, with an estimated 3 to 5 million pounds applied in 2001. First registered for use in the United States in 1974, terbufos was initially used to control insects and nematodes on corn. Currently, terbufos is also registered for use on sugar beets, sorghum, and bananas; terbufos is not registered for residential use or in public health applications. Terbufos is metabolized and activated to its neurotoxic form via oxidative desulfuration. This active form of terbufos irreversibly inhibits acetylcholinesterase, leading to the accumulation of acetylcholine and the classic signs and symptoms associated with muscarinic and nicotinic receptor overstimulation. The United States Environmental Protection Agency (USEPA) classifies terbufos in toxicity category I (high acute toxicity), but as group E for carcinogenicity (evidence of non-carcinogenicity for humans). This group E carcinogenicity classification is based largely on the lack of carcinogenic effects in two animal studies and lack of mutagenicity in several short-term genotoxicity assays. Primarily formulated as granules, Terbufos is applied at planting in a band or directly to the seed furrow. Terbufos is not approved for use in the European Union.