Western flower thrips, Frankliniella occidentali s (Pergande), has developed a high level of resistance to pyrethroid insecticides in Australia. Biochemical examination of those pyrethroid-resistant populations shows esterase activity 2.0- to 6.5-fold higher in six of the seven resistant field populations compared with the susceptible population. This increase in esterase activity in the resistant populations was further supported by native polyacrylamide gel electrophoresis and inhibition studies with the organophosphate insecticide profenofos. The polyacrylamide gel electrophoresis analysis showed differences in the banding pattern and intensity of the esterase isozymes between the pyrethroid-resistant and susceptible F. occidentalis populations. Glutathione S-transferase activity was significantly higher (up to 1.7-fold) in five of the seven resistant field populations when compared with the susceptible populations. Esterases appear to be an important mechanism that contributes to pyrethroid resistance. Glutathione S-transferases may also play a role in pyrethroid resistance in Australian F. occidentalis populations.