Insensitive acetylcholinesterase (AChE) was determined to be basically involved in a EPN-resistant (ER) strain of diamondback moth (DBM, Plutella xylostella L.), as estimated by the AChE inhibition assay using DDVP and thiodicarb as inhibitors in nondenaturing electrophoresis gel. AChEs were clearly separated into four different bands (a major band and three minor bands) in susceptible strain (CS) and all bands inhibited by used inhibitors almost same level, however, only two bands (a major band and a minor band) showed in that of ER strain and major band showed higher insensitivity. Moreover, ER strain showed cross resistance against used inhibitors and DDVP highly inhibited esterase in both strains. About 2kb of ace1,2 cloned and point mutations were detected in ER strain.