This research was performed to find out and characterize the specific esterase isozymes related to OP and pyrethroid resistance in the tobacco cutworm Spodoptera litura(Fabricius). Two laboratory strains (${DSR}_{5}$ and ${CSR}_{4}$) of S.litura, selected with deltamethrin and chlorpyrifos-methyl, had higher larval esterase activities than Haman-collected(HC) strain. Ten esterase isozymes were separated in the wild HC strain on 6.5% nondenaturing polyacrylamide gel electrophoresis (pH 8.3), but only 5 of them were detected in the two strains selected with insecticides. The isozymes were designated from E1 to E10 according to their mobility to cathode. E4 was stained more strongly in both laboratory-selected strains than in HC strain. The frequency of E2 in ${DSR}_{5}$ strain was higher than in HC strain. E2 and E4 were proved to be arylesterase and carboxyesterase. respectively, according to enzyme inhibition tests. Thus decreased number and increased intensity of esterase bands in ${DSR}_{5}$ and ${CSR}_{4}$ strain may be associated with insecticide resistance, resulting from selections successively with insecticides for several generations.