Comparisons of lepidopteran toxicity made among genetic isolines of Monsanto cotton variety 15985, expressing Cry1Ac only, Cry2Ab only, and both toxins, demonstrated a significant increase in activity of the 2-toxin isoline over the Bollgard^® isoline (Cry1Ac-only) with Cry2Ab contributing the larger proportion of activity in the 2-toxin line. Diet overlay studies revealed responses to the 15985 isolines indicating an additive interaction of the 2 Bt toxins in the 2-gene (Bollgard^® II) isoline. A 10-site field study showed that co-expression of Cry1Ac and Cry2Ab resulted in 3 to 6-fold increases in bioactiviy levels over those found in cotton plants expressing Cry1Ac only (Bollgard^®).