A field study was conducted to compare the relative densities, distribution patterns, and economic impact of bollworms, Helicoverpa zea (Boddie), and tobacco budworms, Heliothis virescens (F.), in Monsanto's transgenic cotton ‘BT01DP' expressing the delta-endotoxin from the bacterium, Bacillus thuringiensis subsp. kurstaki (Berliner) (Bt), a 90:10 Bt:non-Bt seed mix, and nontransgenic ‘Coker 312' cotton. These treatments received no applications of insecticides for control of lepidopteran insects. The bollworm was the predominant species, comprising approximately 95% of the bollworm/ tobacco budworm complex. Examination of ten consecutive plants in each of the two center rows of each plot revealed that the 16.7% of plants in the 90:10 Bt:non-Bt plots infested with third-instar and larger larvae was significantly greater than the expected percentage of 9.5. Similarly, the 9.2% of plants in the 90:10 plots infested with fourth-instar and larger larvae was significantly greater than the expected percentage of 4.9. These higher than expected percentages of plants infested with large larvae in the 90:10 Bt:non-Bt plots indicated that movement of larvae from non-Bt to Bt plants occurred. Results indicated that movement of bollworm larvae to Bt plants may result in a significant increase in damage and reduced yield in mixed stands of Bt and non-Bt plants. Additional studies are needed to determine the relation-ships which exist among seed mix ratios, population densities of bollworms/ tobacco budworms, injury, and yield reductions.