Organochlorine, organophosphate (o.p.) and carbamate insecticides have been extensively used in Australia to combatLucilia cuprinaWied. the main initiator of fly-strike of sheep occurring wherever sheep are run on the continent. The improved mules operation, mid-season crutching and insecticides are important in the management of Merino sheep. Insecticides are particularly valuable against body strike, especially in young sheep, in years with intermittent rain during the warmer months. Resistance to larvicides developed in the blowfly over a period of 10y from 1957. The time between the introduction of various insecticides and development of resistance is considered and a comparison made between the emergence of resistance problems to the newer synthetic insecticides in the housefly in Denmark, and in the blowfly in Australia. To provide a complete history of insecticides against the blowfly reference is made to arsenicals, to which there was no suspicion of resistance until low order cross resistance was diagnosed in o.p.-carbamate resistant strains. Some seven years after their introduction DDT and γ BHC were replaced by cyclodiene insecticides in 1954/1955. Reasons are advanced to explain the non-emergence of resistance to DDT and BHC in that period. Resistance to dieldrin and aldrin developed in late 1957 after which diazinon was introduced. The resistance has a typical BHC/dieldrin resistance spectrum and is due to a semi-dominant gene which has persisted in the field in the absence of pressure from cyclodiene insecticides. Non-specific resistance to o.p. insecticides developed in two steps. Low order resistance, diagnosed in 1965, was supplemented by an additional resistance mechanism in 1966, three alleles on two chromosomes are involved. The carbamate, butacarb, was effective against o.p. resistant strains of the blowfly when introduced in the 1966/67 season. In 1967, resistance to butacarb was diagnosed and rapidly became widespread. For the past three fly seasons larvae have been used to monitor resistance levels to o.p. and carbamate insecticides; resistance factors to both these classes of insecticide are significantly higher in larvae than adult females. Resistance levels to o.p. insecticides have stabilised. By contrast resistance levels to butacarb have doubled. A combined o p.-carbamate resistance generally applies in field samples. Larvae from o.p.-carbamate resistant strains form artificial strikes earlier than susceptible larvae, particularly on sheep treated with butacarb. It is concluded that registered o.p. insecticides, but not butacarb, will still give considerable protection against fly-strike, providing the maximum levels of o.p. resistance, reached in laboratory selection programmes, are not exceeded in the field. Investigation into o.p. resistance in the species suggests that the resistance mechanisms place their carriers at a disadvantage in the absence of selection pressure. Measures to minimise the amount of insecticide used against the blowfly are therefore strongly advocated—these include: the improved mules operation, mid season crutching and good animal husbandry.