Three cotton varieties were grown in the field and bioassayed in the laboratory against the beet armyworm (BAW), Spodoptera exigua. A non-B.t. standard, DPL 5415, was compared to NuCotton 33B, which produces the Cry1Ac protein, and NuCotton 33BII, which produces both the Cry1Ac and Cry2Ab proteins. Four leaf ages (3^rd, 6^th, 9^th and 12^th leaf from the terminal) were used in the bioassay to evaluate effects of leaf age. Bioassays were initiated with one day old larvae and run for 10 days or 5 day old larvae and run for 8 days. The bioassays generally showed increased mortality and decreased weight of surviving larvae with increased leaf age for all three varieties. NuCotton 33BII showed excellent activity against BAW. The greatest survival in NuCotton 33BII occurred on the younger leaves, but surviving larvae had grown little or none and were generally about 1/10 the weight of larvae on DPL 5415. It is extremely unlikely that any of the larvae on NuCotton 33BII would have been able to develop to the adult stage.