Studies on insect food intake and utilization are important for determining the degree of insect/plant association and host species’ resistance, and also for helping design pest management programs by providing estimates of potential economic losses, techniques for mass breeding of insects, and identifying physiological differences between species. We studied the feeding and development of fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), on transgenic (Bt) and non-transgenic (non-Bt) cotton. The larvae of S. frugiperda fed on Bt cotton had a longer development period (23.0 days) than those fed on non-Bt cotton (20.2 days). Survivorship of S. frugiperda larvae fed on Bt cotton (74.1%) was lower than that of larvae fed on non-Bt cotton (96.7%). Pupal weight of larvae fed on Bt cotton (0.042 g) was lower than that of larvae fed on non-Bt cotton (0.061 g). The cotton cultivar significantly affected food intake, feces production, metabolization, and food assimilation by S. frugiperda larvae. However, it did not affect their weight gain. Intake of Bt-cotton leaf (0.53 g dry weight) per S. frugiperda larva was lower than the intake of non-Bt-cotton leaf (0.61 g dry weight). Larvae fed on Bt-cotton leaves produced less feces (0.25 g dry weight) than those fed on non-Bt-cotton leaves (0.37 g dry weight). Weight gain per S. frugiperda larva fed on Bt-cotton leaves (0.058 g dry weight) was similar to the weight gain for larvae fed on non-Bt-cotton leaves (0.056 g dry weight). The cotton cultivar significantly affected the relative growth, consumption, and metabolic rates, as well as other nutritional indices: the figures were lower for larvae fed on Bt-cotton leaves than for larvae fed on non-transgenic cotton leaves.