Resistance of insects to insecticides may be due, in part, to behavioral resistance. The resistant insect avoids, or reduces contact with, the insecticide compared to susceptible insects. A study of the behavior of a pyrethroid-resistant strain (R) and a -susceptible strain (S) of Heliothis virescens was conducted on squaring cotton plants in the greenhouse. Treatments were plants sprayed with (1) chlordimeform at the LC(20) for the susceptible strain, (2) cypermethrin at the LC20 for the susceptible strain, (3) a mixture of chlordimeform and cypermethrin at the LC(20) for the susceptible strain, or (4) unsprayed as a control. Third instar R and S larvae were placed on the plants one hour after spraying and their behavior recorded. Feeding of S larvae was significantly reduced on plants treated with cypermethrin compared to R larvae, or S and R larvae on unsprayed plants. Chlordimeform treated plants significantly reduced flower bud feeding, and increased locomotion on leaves for both S and R larvae compared to untreated plants. The mixture of chlordimeform and cypermethrin significantly increased locomotion and reduced feeding of R insects compared to plants treated with cypermethrin only. Both S and R insects more frequently left plants treated with chlordimeform than untreated or cypermethrin treated plants. These results show that chlordimeform can alter behavior of S and R larvae on cypermethrin treated plants. The chlordimeform altered behavior should reduce flower bud damage and increase the potential for larval pick-up of insecticide residues by both strains.