In 1999, field and laboratory assays were performed to compare the relative efficacy of near-isogenic cultivars of cotton (Gossypium hirsutum L.), expressing none, one or two insecticidal proteins of Bacillus thuringiensis Berliner, on various caterpillar pests (Lepidoptera: Noctuidae). Bollworm, Helicoverpa zea (Boddie), and soybean looper, Pseudoplusia includens (Walker), populations were high enough to determine that Bt cotton, expressing both CryIAc and CryX endotoxins of B. thuringiensis, was more effective in controlling these pests than was cotton expressing only CryIAc. CryX is a pseudonym, and the actual ‘Cry' classification of this toxin is unreported. Cotton expressing both toxins is currently being referred to as Bollgard II. Laboratory assays where various plant parts were fed to insects indicated that the dual-toxin Bt cultivar was more toxic to bollworm, fall armyworm (Spodoptera frugiperda [Smith]) and beet armyworm (Spodoptera exigua [Heubner]) than the single-toxin cultivar. It appears that the newly-developed stacked Bt cultivars will be more effective and have a wider range of activity on the various caterpillar pests commonly encountered in cotton. The implications of this improved activity are addressed.