The objective of this investigation was to genetically engineer cotton plants that would be resistant to major Lepidopteran pests. Using mediated transformation, a modified version of the Bacillus thuringiensis delta-endotoxin (B. t.) gene was introduced into cotton cells. Intact plants were regenerated from the transformed cell lines. Based on Southern blot analysis, most of the regenerated plants carried at least one intact copy of the B. t. gene. Insect feeding trials using Heliothis zea (cotton bollworm) and Trichoplusia (cabbage looper) showed that some of the B. t. containing plants were capable of killing as well as inhibiting feeding by these insects. The level of toxicity of independently-derived plants varied, probably resulting from the random nature of gene insertion. Progeny analysis indicated that the insect resistance trait was inherited in a Mendelian fashion according to the number of copies of the B. t. gene present.