Ten strains of red flour beetles, Tribolium castaneum (Herbst), collected from 1963 to 1965 from storage facilities in which malathion had been used for various known periods were tested for resistance to topical applications of malathion. The most resistant strain required 11.3 times as much malathion for the LD50 as did the susceptible standard laboratory strain. Insects having no known exposure to malathion in warehouses were nearly as susceptible as the laboratory strain.