Laboratory bioassays were carried out to determine susceptibility levels of pink bollworm (PBW) populations from four cotton production regions of Mexico to the Bt endotoxin CryIAc. Bt cotton performance was evaluated by sampling bolls and quantifying PBW infestations in four locations of the Comarca Lagunera, Mexico. PBW populations from the Comarca Lagunera, Jiménez and Delicias, Chih., and Mexicali, B. C. are susceptible to the endotoxin CryIAc. The PBW population from Mexicali, B. C. was the most susceptible to the toxin. PBW larvae developed on bolls were more susceptible to the toxin than those developed on squares. Larvae growth (size and weight) was drastically reduced by toxin concentrations of 0.1 and 0.3 g/ml. Bt cotton is highly effective against the PBW in the Comarca Lagunera, which has caused a significant reduction in the number of insecticide applications.