Three house fly, Musca domestica L., strains, two from farms near Guelph, Ontario and one from a farm near Edmonton, Alberta were cultured and tested for their resistance to insecticides as compared with a susceptible laboratory strain. The two Guelph strains were resistant to the 7 organochlorine (OC) and 11 organophosphorus (OP) insecticides tested and to nearly all of the carbamate insecticides. One Guelph strain (A) which had been subjected to minimum pyrethrins pressure and no residual pyrethroid pressure was susceptible to pyrethrins and the 8 pyrethroid insecticides tested. The other Guelph strain (B), which had been subjected to intense pyrethrins–pyrethroid pressure, was resistant to pyrethrins and to all pyrethroids tested. The Edmonton strain had a history of insecticide exposure similar to the Guelph B strain and showed a similar resistance pattern. Resistance levels at the LD50 for the Guelph B strain, as compared with a susceptible laboratory strain, for the OP insecticides tested, ranged from ×50 for malathion to ×4.5 for dichlorvos; for the carbamates, from ×13.5 for bendiocarb to ×4.1 for methomyl; and for the pyrethroids, from ×55.6 for deltamethrin to ×18.7 for fenvalerate.