Four strains of adult red flour beetles, Tribolium castaneum (Herbst), and six strains of adult sawtoothed grain beetles, Oryzaephilus surinamensis (L.), infesting barley stored on farms in Minnesota were surveyed for resistance to malathion, pirimiphos-methyl, and chlorpyrifos-methyl. Adults were exposed to filter papers impregnated with each of the insecticides. Dose-response and discriminating-dose test procedures were used to evaluate malathion resistance in field strains. Only the discriminating-dose method was used to evaluate pirimiphos-methyl and chlorpyrifos-methyl resistance. Dose-response and discriminating-dose tests indicated malathion resistance in all field strains of T. castaneum , but not in O. surinamensis field strains. T. castaneum strains did not show cross-resistance to pirimiphosmethyl or chlorpyrifos-methyl. All six O. surinamensis strains were susceptible to pirimiphosmethyl, but four strains showed reduced susceptibility (tolerance) to chlorpyrifos-methyl. The use of synergists in combination with malathion on T. castaneum strains and with chlorpyrifos-methyl on the four O. surinamensis strains indicated that carboxylesterase was involved in detoxifying malathion in resistant T. castaneum adults, whereas oxidases and esterases other than carboxylesterase were involved in detoxifying chlorpyrifos-methyl in tolerant O. surinamensis adults.