The effect of chlordimeform (CDF) on permethrin and cyhalothrin-K uptake from a treated surface by third instar larvae of the tobacco budworm was determined. In general, CDF increased the uptake of both pyrethroids, especially in pyrethroid resistant (ICI-R) tobacco budworms. Compared to the susceptible (LSU-Lab) tobacco budworm larvae, the pyrethroid resistant strain consistently picked up less insecticide. A hot-probe bioassay for knock-down resistance (kdr) (Bloomquist & Miller 1985) indicated the presence of kdr in the pyrethroid resistant strain. The pyrethroid-resistant strain also had higher titers of trans-permethrin hydrolabe activity. The role of metabolism in pyrethroid resistant tobacco budworm larvae was evaluated by topically treating third instar larvae with C(12) radiolabeled permethrin-or cyhalothrin-K. At 18 hr posttreatment, the resistant larvae treated with permethrin had less total radiolabeled material internal, and less parent than did the susceptible larvae. However, for cyhalothrin-K treated larvae there was little difference between the two strains in the amount of total internal radiolabel or in the amount of parent. Thus, there appears to be some differences in how the resistant strain handles permethrin and cyhalothrin-K. The above studies suggest that kdr, an increased metabolic capability and, perhaps, a reduced level of larval activity in the presence of the pyrethroids all contribute to pyrethroid resistance in the ICI strain of the tobacco budworm.