Laboratory bioassays were conducted to evaluate mortality and development of the corn earworm, Helicoverpa zea (Boddie), fed on meridic diet with different incorporated concentrations of the Cry1Ac endotoxin of Bacillus thuringiensis. The basic insect colony was obtained from maize of the Colombian “Piedemonte” eastern plains. LC₅₀ calculated by Probit analysis seven days after starting the experiments with neonate larvae, varied from 3.45 to 6.12 and LC₈₀ from 7.12 to 10 µg/ml. Toxin concentration had a marked influence on larval and pupal development, expressed as weight decrease as toxin concentration increased in the diet. Concentrations near or higher than 1.0 ppm caused high larval mortality and a high percent of deformed pupae. No significant difference between the weights obtained with low dosages and the check was obtained. However, a significant increase of the developmental time of both stages was observed when larvae were fed on diet with concentrations higher than 0.01 µg/ml, meaning a longer feeding period and fever annual generations of this polyphagous insect. It is considered that these basic data represent valuable information on the developmental responses of the insect exposed to transgenic cotton and corn, expressing the Cry1Ac toxin.