Neonate and third-instar larvae of susceptible and pyrethroid-resistant tobacco budworm (TBW), Heliothis virescens (F.), were bioassayed by exposure to residues of insecticides in glass liquid-scintillation vials. With neonate larvae, cross-resistance was observed to all pyrethoids, but not to other classes of insecticides (Campanhola & Plapp 1987). In this study, no crossresistance was observed to the combinations chlorpyrifos plus sulprofos, cypermethrin plus chlordimeform and piperonyl butoxide, and cypermethrin plus non-pyrethroid insecticides (profenofos, methyl parathion or methomyl). Chlordimeform synergized all insecticide combinations against resistant neonate larvae. The widespread resistance to the pyrethroids, the absence of resistance to organophosphates (Campanhola and Plapp 1988a), and the low synergism of cypermethrin by piperonyl butoxide are evidence for the presence of target-site resistance in neonate larvae. Effective insecticides and insecticide combinations for the control of pyrethroid-resistant third instar TBWs included profenofos, sulprofos, acephate (this can be combined with chlordimeform), endosulfan, possibly methomyl and thiodicarb, combined or not with chlordimeform, and the insecticide combinations cypermethrin plus methyl parathion and cypermethrin plus thiodicarb, with or without chlordimeform. A high level of cypermethrin resistance in third instars, resistance to methyl parathion, and synergism of these insecticides by piperonyl butoxide are evidence for the presence of metabolic resistance in this life stage. Therefore, target-site resistance to pyrethroids appeared to be equally manifested in all developmental stages of the TBW while metabolic resistance was mostly expressed in third instars. Biological differences were found between the susceptible and pyrethroid-resistant strains. Differences included longer developmental period, reduced egg production, and lower number of females producing offspring in the resistant (ICI) strain. Thus, pyrethroid-selected individuals may not be able to compete reproductively with susceptible insects, resulting in an increased pyrethroid susceptibility in the absence of insecticide exposure. This aspect can be exploited in resistance management programs.