Susceptibility to fenitrothion, carboxylesterase activity (substrate:α-naphthyl acetate) and carboxylesterase isozyme pattern in the rice stem borer, Chilo suppressalis Walker, collected from Yamagata Prefecture were studied. Among four strains tested, three strains showed high levels of fenitrothion resistance, and there was a positive relationship between the resistance and carboxylesterase activity. Isoelectrofocusing revealed that a single carboxylesterase isozyme (pI=5.2) was preferencially observed in the three resistant strains.