Cytochrome P450s (P450s) are known to oxidize a variety of insecticides including pyrethroids, thereby conferring metabolic resistance in diamondback moth (DBM), Plutella xylostella. Synergism assay with piperonyl butoxide indicated that the enhanced activity of P450 is associated with pyrethroid resistance in a cypermethrin-resistant (CR) strain. However, there were little differences in the basal transcription levels of all the P450s examined between susceptible (Sus) and CR strains, suggesting that constitutive overexpression of P450 is not likely involved in the cypermethrin resistance but induction of P450 by cypermethrin is rather associated with metabolic resistance. To determine the conditions resulting in maximum levels of P450 induction, several factors including the way of adminstration (topical application vs. leaf dipping), exposure dose and exposure duration were examined. In general, leaf dipping method resulted in greater levels of induction in a wider array of P450s. The conditions of ‘low dose (sublethal dose or concentration) and short exposure (less than 3 hr)' to cypermethrin were more efficient in P450 induction than those of ‘high dose (around LD50 or LC50) and long exposure (more than overnight)’, which have been employed in many other studies to date. Cross-strain comparison revealed that 9 of 11 P450s were induced 1.4-2.2 fold in CR whereas only 3 P450s in Sus under the optimal induction conditions, demonstrating that metabolic resistance in CR strain is actually conferred by the mechanism of selective P450 induction when exposed to cypermethrin.