Susceptibility of beet armyworm, Spodoptera exigua Hübner, to the Bt toxin, Cry1Ac, expressed in first generation of transgenic cotton plants was evaluated using a laboratory strain and several foreign and United States field strains. A diet-incorporation assay of neonate larvae was used. Susceptibility was estimated by the degree to which Cry1Ac inhibited larval growth from the first to fifth instar. Regression analysis of larval weights against log concentration of Cry1Ac yielded slope and intercept values that were used to compute I _50s, defined as the amount of Cry1Ac that resulted in a fifty percent reduction in larval growth. Three populations exhibiting reduced susceptibility to Cry1Ac were selected on diet containing 1000 micrograms of Cry1Ac per gram of diet. I _50s for non-selected populations ranged from 0.0477 micrograms Cry1Ac per gram of diet for the laboratory reference strain to 4.31 micrograms of Cry1Ac per gram of diet for a field strain collected from Yuma, Arizona. Selection of a strain from Belle Glade, Florida, with Cry1Ac yielded the lowest susceptibility to this toxin. Prior to selection, the I ₅₀ was 2.43 micrograms of Cry1Ac per gram of diet; after selection the I ₅₀ was 17.4 micrograms of Cry1Ac per gram of diet. Thus selection reduced susceptibility of the Belle Glade, Florida strain to Cry1Ac by 7.2-fold and yielded susceptibility that was 360-fold less than the laboratory reference strain. Selection also reduced susceptibility of an Arizona (Dome Valley) population by 3-fold. Our results demonstrate the presence of large (>25-fold) differences in susceptibility of Arizona beet armyworm populations to Cry1Ac. Furthermore, the fact that resistance was elevated three to seven-fold in two selected strains provided evidence of a genetic basis of resistance to Cry1Ac in S. exigua.