Five contemporary strains of the bollworm Helicoverpa armigera Hubner from China, Pakistan and India, all with high resistance to pyrethroids, were compared with a standard susceptible strain that originated from the Cote D Ivoire in the 1970s (SCD). Two of the Chinese strains (YGF and YGFP) were derived by laboratory selection from a third, field collected strain (YG). The strain YG exhibited 7-, 14- and 21- fold resistance to fenvalerate, cypermethrin and deltamethrin, respectively. After selection with fenvalerate for 14 generations (YGF), this increased to 1690-, 540- and 73-fold. Selection with a mixture of fenvalerate and piperonyl butoxide (PBO) for 14 generations (YGFP) resulted in resistance ratios of 2510, 2920 and 286. The synergistic ratios to fenvalerate that resulted from pre-treatment of PBO were 5-, 462- and 12-fold in YG, YGF and YGFP strains, respectively. Resistance ratios for a Pakistani strain (PAK) were 2320-, 4100- and 223-fold to fenvalerate, cypermethrin and deltamethrin, respectively. The synergistic ratio of PBO to these pyrethroids was 450-, 950- and 11-fold. The strong synergism of pyrethroids by PBO implied that an oxidative metabolism could be involved in pyrethroid resistance in these resistant strains. The activities of cytochrome P450 monooxygenases from midguts of final instar larvae to p-nitroanisole (PNOD), ethoxycoumarin (ECOD), methoxyresorufin (MROD) significantly increased in all the resistant strains when compared with the susceptible strain. This further implies that cytochrome P450 monooxygenases are involved in pyrethroid resistance in Asian H. armigera. Comparative in vitro studies of the metabolism of 14C-deltamethrin by midgut microsomes of the resistant PAK and susceptible SCD strains showed that the resistant strain had a much greater capacity than the susceptible strain for the metabolic degradation of deltamethrin. This enhanced metabolic degradation occurred in the presence of NADPH which suggested an oxidative detoxification. In the resistant strains, minor increases in glutathione S-transferase activity to the substrates (CDNB and DCNB), and esterase activity (to the substrate a-naphthyl acetate) further suggested that, of the putative metabolic mechanisms, oxidases are the most important. This study provides the first evidence that cytochrome P450 monooxygenases are a major metabolic mechanism responsible for pyrethroid resistance in H. armigera from Asia.