Induced resistance in cotton plants to herbivory by the cotton bollworm (Helicoverpa zea) was studied. Induced resistance was indicated by decreased larval growth of H. zea when larvae fed on previously damaged foliage or squares compared to the undamaged controls. Herbivory caused a significant increase in several primary gene products including peroxidase, ascorbate oxidase, lipoxygenase and diamine oxidase in foliage or squares. The exogenous application of methyl jasmonate on cotton plants also elicited resistance against H. zea. Our findings suggest that enhanced resistance to the bollworm may be achieved via genetic amplification of several defensive genes and/or by application of chemical elicitors of resistance.