Pharmacokinetic and physiological assays were used to determine mechanisms associated with pyrethroid resistance in Heliothis virescens (F.) collected from a single cotton growing region in Louisiana during the four months of the growing season. Reduced neuronal sensitivity was detected in larvae from an early-season (June) collection, and occurred with reduced frequencies in populations sampled later in the season. Enhanced metabolism of cypermethrin was measured in larval insects and increased in frequency within populations sampled in mid- to late-season. Adult insects also expressed levels of cypermethrin metabolism and excretion that were higher than those measured in a laboratory, reference strain, but the relevance of this result to pyrethroid resistance is not clear because enhanced levels in these insects were not associated with decreased mortality. These results indicate that multiple mechanisms are associated with pyrethroid resistance in this insect, and that levels of expression of these mechanisms in populations of H. virescens may fluctuate during the growing season.