A field study was conducted in 2000 to evaluate insect population dynamics and the efficacy of cotton (Gossypium hirsutum L.) varieties expressing none, one or two Bacillus thuringiensis proteins on various chewing pests. Overall, insect pest populations were very low. Only aphid populations reached a treatable threshold. Late bollworm damage was significantly higher on the non- Bt variety, but that did not significantly reduce lint yields. Less looper feeding damage was observed on the two- Bt protein variety during the initial infestation. The one- Bt varieties keep looper populations in check while feeding continued on the non- Bt variety. Leaf damage did not have a negative effect on lint yield.