The activity of dibrom as a synergist of organophosphate and pyrethroid toxicity was evaluated in laboratory strains of the tobacco budworm, Heliothis virescens (F.). In the first series of experiments, a maximum non-lethal dose of dibrom was applied topically to the abdominal dorsum of fifth instars, then 30 minutes later, a dose corresponding to the LD₅₀ of either cypermethrin or profenofos was applied to the midthoracic dorsum. In tests with an insecticide-susceptible strain, dibrom synergized the toxicity of cypermethrin (synergism ratio= 1.76), but not profenofos. In contrast, in bioassays with dibrom and insecticide-resistant larvae, no synergism of cypermethrin toxicity was detected, but dibrom significantly synergized the toxicity of profenofos in larvae from strains resistant to organophosphates and pyrethroids (synergism ratios= 1.46 and 2.89, respectively). Similar results were measured in bioassays where an LD₅₀ of dibrom was co-applied with low doses of either cypermethrin or profenofos. These findings confirm previous results suggesting that resistance is esterase-based in these strains, and illustrate the potential for use of organophosphates as synergists of pyrethroid or organophosphate toxicity in insecticide-resistant strains of the tobacco budworm.