Notwithstanding that Colombia, with the planting of cotton which has the Bacillus thuringiensis toxin CrylAc incorporated, has entered the era of transgenic cultivars, a deficiency of basic studies required for an adequate and opportune management of these crops and their insect pests exists. The lack of knowledge of the response of omnivorous insects, such as the fall armyworm (FAW), Spodoptera frugiperda, pest of both cotton and corn, and which behavior regarding Bt cotton has not been studied in the place of origin of the variety, stands out. Bt toxins can cause larval mortality or influence negatively the development of the Ufe cycle of the insect, effects which were intended to determine feeding the FAW larvae with fresh tissue of transgenic cotton and with an artificial diet with incorporated CrylAc. The results showed that the toxin incorporated within the transgenic cotton would not cause a mortality of the pest under field conditions, but influence the development of the life cycle, increasing it. Furthermore, a significant decrease in pupal weight was observed, which however did not interfere with the females fertility and fecundity.