Acetylcholinesterase and carboxylesterase activities of four strains of Oryzaephilus surinamensis (L.) larvae were examined. The three strains of VOS49, VOSF, and VOSCM were resistant to malathion, fenitrothion, and chlorpyrifos-methyl, respectively, while VOS48 was a susceptible strain. Cross-resistance to all three organophosphate insecticides tested was confirmed in larvae of the three resistant strains. Acetylcholinesterase activity was not correlated to organophosphate resistance in those resistant strains. VOS49, VOSF and VOSCM showed enhanced levels of carboxylesterase activity based on p-nitrophenyl acetate, $alpha$-naphthyl acetate, or $eta$-naphthyl acetate substrates. However, these activities were significantly lower than those of O. surinamensis adults. PAGE zymograms showed major differences in carboxylesterase band pattern among strains. VOS49 carboxylesterase banding pattern was significantly different from other two resistant strains. Interestingly, these larval esterase band patterns were very similar to those of O. surinamensis adults. Therefore, carboxylesterase activity of O. surinamensis plays important role in resistance to organophosphate insecticide from larval stage.