Offspring from a field strain of house flies taken from the L.S.U. poultry farm in the Baton Rouge, Louisiana area in October 1957 were found by topical application to be highly resistant to malathion and DDT with varying degrees of resistance to the other insecticides tested. Malathion had been used exclusively for 3 years. Progeny from field strains taken from the same area in October and December 1958 were more resistant to malathion, perhaps due to continued use of the organic phosphates. In the resistant group the larval period was longer and pupation and emergence extended over a longer time interval than the Orlando, Florida susceptible strain.