Electrophoresis was used to characterize the esterases in Lucilia cuprina (Wiedemann). Of the 16 esterase bands visualized, only one was associated with resistance to organophosphorus insecticides. In laboratory reference strains, the esterase E3 was consistently found in both susceptible and heterozygously resistant flies, but was absent from homozygously resistant flies. It was postulated that resistant flies possessed a non-staining form of E3, designated E3null. Genetic analyses mapped the locus for E3null to a position in the same region as the gene for organophosphorus resistance. No recombinants between the genes for resistance and E3null were detected. In field populations of L. cuprina from several areas in Australia, a close association was found between the frequency of E (0·97) and the proportion of flies resistant to organophosphorus insecticide (0·967). This association suggests that E3null represents the product of a major resistance gene in terms of the ‘mutant ali-esterase’ theory. E3 as found in susceptible blowflies, represents the ‘original’ ali-esterase, the gene for which mutated to one coding for E3null. This mutant enzyme can hydrolyse organophosphates more efficiently than E3 but has lost the ability to hydrolyse 1- and 2-naphthyl acetate (the substrates used to visualize the esterases after electrophoresis).