Very high cypermethrin and fenvalerate resistance frequencies were recorded in Helicoverpa armigera (Hübner) populations in central India during the 1993–94, 1994–95 and 1995–96 cropping seasons. Synergism assays and biochemical analyses of detoxification enzyme levels indicated that mono-oxygenases and esterases were important metabolic mechanisms mediating pyrethroid resistance. Piperonyl butoxide- (PBO) and profenofos-suppressible pyrethroid resistance were correlated with enhanced levels of cytochrome P450 and general esterases respectively. Enzyme assay data indicated that high cytochrome P450 levels generally coincided with low esterase activity and vice versa. Similarly, synergist bioassays showed that PBO-insensitive resistance was frequently associated with profenofos-sensitive resistance and vice versa. Oxidase- and esterase-mediated mechanisms evidently alternated in a reciprocal manner, with perceptible shifts in relative importance occurring during mid-October in all three seasons and in late January in 1995. Apart from metabolic mechanisms, a synergist-insensitive resistance mechanism (believed to be nerve insensitivity), accounted for an average of 51, 30 and 28% of cypermethrin resistance during the 1993–94, 1994–95 and 1995–96 seasons respectively.