Susceptibility to some pesticides of the diamondback moth, Plutella xylostella L., collected from Wakayama Prefecture, Japan was examined with the third instar larvae by the leaf-dip method. Most populations under test showed remarkably reduced susceptibility to organophosphates and carbamates. Cartap was relatively effective except in some locations. Pyrethroid resistance was first identified in Wakayama, Gobo and Inami, where cruciferous crops were extensively grown, and the resistant moths spread over Wakayama Pref. with time. Most of the populations showed no resistance to Bacillus thuringiensis (BT). The Gobo population however developed resistance to BT, reaching approximately 60 of resistance ratio in 1986 and 1987 under the frequent applications of BT. No resistant populations for chitin synthesis inhibitors have been observed so far.