Eggs and larvae of L. cuprina were coIJected from natural fly strikes in a flock of Merino ewes in which sheep had been either treated with the insecticide dieldrin or left as controls. An analysis of gene and genotype frequencies of Rdl locus, which determines resistance to dieldrin, provides support for the existence of strong selection operating during larval development on sheep whose fleece contain insecticide residue. Resistance genotypes appear to be at a disadvantage both in the laboratory and in the insecticide-free environment of control sheep. There is no evidence that flies of different resistance status choose oviposition sites on the basis of the presence of dieldrin residues in the fleece.An explanation is provided for the observation that natural selection for insecticide resistance in the sheep blowfly utilizes major locus variation although the response to laboratory selection is polygenic in origin. The parameters determining the evolution of insecticide resistance in L. cuprina on sheep are discussed.