In vitro esterase and monooxygenase activities were measured in field strains of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.), differentially resistant to fenitrothion, chlorpyrifos-methyl, and pirimiphos-methyl. Aldrin epoxidase and NADPH-cytochrome c reductase activities and cytochrome P450 and b(5) levels were elevated in all resistant strains. These results suggest that fenitrothion resistance in each strain can be attributed largely to the monooxygenase system. Esterase activities toward alpha- and beta-naphthylacetate, p-nitro-phenylacetate, methylthiobutyrate, and phenylacetate were correlated with resistances to chlolpyrifos-methyl and pirimiphos-methyl. Correlations with chlorpyrifos-methyl resistance were particularly marked when only strains showing low-moderate resistance levels were considered. The strain showing the highest levels of chlorpyrifos-methyl resistance (along with only moderate resistance to fenitrothion) also showed the highest aldrin epoxidase activity. Whereas low to moderate level resistance to chlorpyrifos-methyl was related closely to esterase activity, the high level resistance shown by this recently collected strain appears to involve both esterase- and monooxygenase-mediated mechanisms. Similarly, the relationship between esterase activity and resistance to pirimiphos-methyl is likely to account for only a part of the resistance. The implications for resistance management are discussed.