Frequencies of profenofos resistance and enzymes activities associated with metabolism of organophosphorus (OP) insecticide were measured in larvae of the tobacco budworm, Heliothis virescens (F.) that were field-collected from Northeast Louisiana during the 1995 and 1996 cotton growing seasons. High frequencies of profenofos resistance were recorded in all field-collected strains and expression of resistance was strongly correlated with esterase (EST) activity toward a-naphthyl acetate but not P450-monooxygenase (P450-MO) activity toward p-nitroanisole. Moderate correlations were measured between profenofos resistance and glutathione S-transferases (GST) activity toward 1-chloro-2,4-dinitrobenzene (CDNB), but not 1,2-dichloro-4-nitrobenzene; DCNB. Finally, there was no association between profenofos resistance in all field strains and sensitivity of acetylcholinesterase (AChE) to inhibition by chlorpyrifos oxon, although in some strains, mean levels of sensitivity were reduced relative to that of an insecticide-susceptible strain. These results suggest that rapid assays measuring EST activity toward a-naphthyl acetate may be useful for monitoring metabolic resistance to OP insecticides in tobacco budworms.