Fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is an important pest of several crops in the western hemisphere. This lepidopteran pest has diverged into two host strains that are partially adapted to maize, Zea mays L., and rice, Oryza sativa L. Both strains are identical morphologically but genetically differentiated by molecular markers. Hosts used by the maize strain include maize, cotton, Gossypium hirsutum L., and sorghum, Sorghum bicolor (L.) Moench, and by the rice strain, rice fields and pasture grass. In the U. S., the corn (maize) strain is less susceptible than the rice strain to the insecticides carbaryl, diazinon, Cypermethrin, methyl parathion, and methomyl, whereas in Colombia the rice strain is more tolerant to the insecticides lambdacyhalothrin and methomyl. Larvae from maize and rice fields from Tolimaa (central Colombia) were genotyped and evaluated for susceptibility to Cry1Ac and Cry1Ab endotoxins of Bacillus thuringiensis Berliner to test whether both populations differed in susceptibility to the two endotoxins. Previous studies obtained from the U. S. have demonstrated that the rice strain was more susceptible to Cry1Ac. After four generations, our results showed the rice strain was more susceptible than the maize strain to both endotoxins. Moreover, in both strains susceptibility to Cry1Ab was stronger than for Cry1Ac. The realized heritability of susceptibility was greater for the rice strain than for the maize strain. The number of generations needed for 10-fold increase in tolerance ranged from 15 to 20 for Cry1Ac and from 47 to 560 generations for Cry1Ab. The genetic basis of susceptibility to both endotoxins has shown that the inheritance of this trait is not additive.