Fall armyworm, Spodoptera frugiperda (J.E. Smith), is the most important economic agricultural pest of the American continent. In Colombia, one of its main plant hosts is cotton (Gossypium hirsutum L.), including those that are genetically modified (Bacillus thuringiensis-expressing (Bt) cotton). To understand the sublethal effects on the demographic parameters of adults, complete life tables were constructed when larvae fed on one of four cotton cultivars: Cry1Ac (NuOpal RR, Bt cotton), Cry1Ac Cry2Ab (DP141 B2RF, Bt cotton), non-Bt (Delta Opal RR), and non-Bt (Oro Blanco). As much as 3% larval survivorship was obtained from Cry1Ac Cry2Ab cotton, 35% in Cry1Ac, compared with 58% in Oro Blanco and 53% in Delta Opal RR. The net reproductive rate (Ro) in Cry1Ac Cry2Ab was 82 and 64 times less than what was obtained from Oro Blanco and Delta Opal RR. The Ro of Cry1Ac was three and two times less than in Oro Blanco and Delta Opal RR. Doubling time (Dt) was 4.4 and 4.2 times longer in Cry1Ac Cry2Ab than in Oro Blanco and Delta Opal RR. No significant differences in Dt were found among Cry1Ac and the non-Bt cultivars. The antibiosis effect of Bt cottons had a strong impact on the demography of S. frugiperda adults. The results complement our understanding of susceptibility to Cry1Ac and Cry2Ab proteins in strains of the pest from different areas of Colombia, and the resulting effects on populations of fall armyworm.