A greenhouse study was conducted to quantify behavior of tobacco budworm, Heliothis virescens (Fabricius), larvae on chlordimeform-treated cotton plants. Compared to untreated plants, chlordimeform (1) reduced plant-feeding by larvae, (2) increased movement by larvae over plant surfaces, especially over leaves and stems, and (3) increased abandonment of plants by larvae. It is hypothesized that under field conditions, insecticide synergism by chlordimeform against Heliothis spp. may be partly due to chlordimeform-induced modification of this pest's behavior.