Populations of L. cuprina collected from adjacent sheep-grazing areas and rubbish tips in Victoria (Mansfield and Warrnambool) and New South Wales (Lismore) were tested for resistance to the insecticides diazinon and dieldrin. Populations from sheep-grazing areas had a significantly higher diazinon Rop-1 allele frequency than those from adjacent tips with the Victorian populations being more resistant than those from Lismore. Victorian sheep and tip populations had similar gene frequencies at the dieldrin resistance locus, but the Rdl allele frequency was significantly greater in the population at the tip than in the population from the sheep-grazing area at Lismore. The Rdl allele is at a higher frequency in flies from the Lismore area than in Victorian populations. The results at both loci are explained by a balance of selection and gene flow between sheep and tip populations and by selective differences between geographical areas. The exceptionally high frequency of the dieldrin Rdl allele in populations at the Lismore tip may be partially explained by the use of dichlorvos for fly control. Dosage mortality curve and genetic analyses suggest that dichlorvos (an organophosphorus compound) may select at the dieldrin resistance locus. Possible mechanisms for this are discussed. The consequences of genetic differentiation between L. cuprina populations within a region for an autocidal control program are considered.