This study has investigated the role of esterases in conferring pyrethroid resistance in Australian populations of the cotton bollworm, Helicoverpa annigera. To achieve this, studies were performed at the population, individual, biochemical and genetic levels. Using native polyacrylamide gel electrophoresis (native PAGE) esterase patterns and levels were assessed in resistant field and laboratory strains and in a susceptible laboratory strain. Esterases were assigned the standard nomenclature of regions A to F, based on the relative mobilities of groups of esterases on native PAGs. The esterases previously associated with pyrethroid resistance and further potential esterases were identified by native PAGE and through comparisons of larvae surviving and those killed by pyrethroid exposure. Resistant field strains showed a general increase in esterase expression and a higher frequency of individuals with additional esterases with relative mobilities between 0.24 and 0.3. Populations from eastern Australia were characterised by native PAGE for esterase expression and were also measured for frequencies of pyrethroid resistance. Frequencies of pyrethroid resistance among populations were not significantly different. However, frequencies of pyrethroid resistance in adult females in some populations were significantly higher than in adult males. Other populations showed more equal frequencies of resistance between the sexes. In collaborative work, levels of cytochrome P450, CYP6B7, mRNA expression in resistant and susceptible larvae were measured by northern blot analysis. Esterase and CYP6B7 expression were not correlated with pyrethroid resistance