Abstract: The LD₅₀，α-naphthylacetate（α-NA）esterase kinetics，α，β-NA esterase activity and IC₅₀ of dichlorvos（DDVP），phoxim and malathion to Del-R、2Cl-R and susceptible (SP) strains of houseflies were determined. The resistances of the Del-R and 2Cl-R to DDVP, phoxim and malathion were 0.966～7.190 folds as much as those of the SP strain，indicating a low resistance level to organophosphate insecticides in the pyrethroid-resistant houseflies. An obvious correlation of the carboxylesterase activity with IC₅₀ suggested that the carboxylesterase would play a significant role in the resistance to organophosphates in pyrethroidresistant houseflies.

Key words: housefly, esterase, carboxylesterase, organophosphate, insecticides