The utility of microplate and electrophoretic assays for detection of metabolic mechanisms of resistance to profenofos in the tobacco budworm, Heliothis virescens (F.) was assessed. Mean esterase (EST) activities were significantly higher in profenofos-resistant than -susceptible larvae and activities were correlated with resistance to profenofos. In addition, qualitative and quantitative variation was observed in electrophoretic gels stained with_a-_and b- naphthyl acetates. Staining of ESTs was more intense with resistant larvae than those from a susceptible strain, and an esterase was detected in 100% of individuals from a profenofos-selected strain but was absent in the susceptible individuals examined. The appearance of this esterase in resistant larvae coincided with the decreased expression of a second esterase in susceptible insects. A similar pattern also was observed in larvae from field-collected strains.