Abstract: Synergism of abamectin(AVMs) by piperonyl butoxide (PB) and TPP in both AVMs-re sistant(AV-R) and susceptible(XH-S) strains of diamondback moth ［DBM, Plute lla oxylostella (L.)］ was investigated. Both PB and TPP increased the tox icity of AVMs to AV-R strain and the synergism ratios are 8.2- and 5.5-folds, respectively, but they all have no effect on the toxicity of AVMs to XH-S stra in. It suggested that the high resistance to AVMs in DBM is correlated to carbox ylesterase and mixed functional oxidase(MFO), which was verified by the comparis on of cytochrome P450 content between these two strains. The cross-resistance o f AVMs to pyrethroid insecticides and insect growth regulators was also studied in AVMs-selected strains. AVMs-selected strains possessed the low cross-r esistance between four pyrethroid insecticides tested and abamectin, but no cross-resistance to two insect growth regulators, flufenoxuron and chlorfluazuron.