Two types of housefly acetylcholinesterases (AChEs) were expressed in a baculovirus lepidopteran cell system to elucidate the effect of point mutations from Ile₁₆₂, Gly₃₄₂ and Phe₄₀₇ in the organophosphate susceptible strains to Met₁₆₂, Ala₃₄₂ and Tnyr₄₀₇ in the organophosphate resistant strains. Concentrations resulting in 50% inhibition (I₅₀) of the expressed enzyme by an organophosphate, fenitroxon, with Ile₁₆₂, Gly₃₄₂ and Phe₄₀₇ was 4.02×10⁻⁷ M, and that of the AChE with Met₁₆₂, Ala₃₄₂ and Tyr₄₀₇ was 9.00×10⁻⁶ M. The ratio of I₅₀ values was 22.4, and was almost equivalent to the ratio of the inhibition assay of homogenates of housefly bodies from which the cDNAs encoding the housefly AChEs were derived. The insensitivity of the housefly AChE to fenitroxon in the insecticide resistant strains is attributed to point mutations in the structural gene.