The inhibition of hydrolysis of acetylcholine phenyl acetate, tributyrin and methyl butyrate by house flies (Musca domestica L.) has been studied at various times after poisoning by four organophosphates: parathion, malathion, Diazinon® (O, O-dicthyl O-2-isopropyl-4-methyl-6-pyrimidinyl phosphorothioate) and Co-ral® (O, O-diethyl O-3-chloro-4-methyl-2-oxo-2H-1-benzopyran-7-yl phosphorothioate). In surviving flies, cholinesterase was invariably the least inhibited enzyme. But in killed flies, cholinesterase was usually the most inhibited enzyme, being 75% to 99% inactivated. The difference between enzyme levels of killed and surviving flies at any time was also greatest for cholinesterase. It is concluded that death is associated with, and probably due to, cholinesterase inhibition.