Spray table tests with cotton (Gossypium hirsutum L.) leaves demonstrated that petroleum diluents and solvents caused higher mortality in laboratory susceptible tobacco budworm, Heliothis virescens (L.), larvae than in a laboratory resistant strain when exposed to the various residues. Paraffinic diluents with bifenthrin caused higher mortality of susceptible and resistant H. virescens larvae under conditions of long-term exposure to spray residues than bifenthrin in isoparaffinic and aromatic solvents. None of the bifenthrin-oil combinations increased mortality above the water-bifenthrin treatment in bioassays of either resistant or susceptible strains.