Base line information was generated on tobacco budworm populations from Mexico to the Cry IA (c) toxin, expressed in the BOLLGARD cotton. This Bacillus thuringiensis transgenic cotton is growing in importance in Mexico, and it is important to have basic information on the response of populations selected by this toxin. Bioassays were carried out on field-collected populations of TBW from two agricultural areas in northeastern and one from northwestern Mexico. They were compared to a laboratory susceptible population. Overlay concentrations of the toxin were applied on lepidoptera diet and data on mortality, larvae reaching 3^rd instar and percent growth inhibition were obtained 7 days after treatment. Treated larvae were maintained under laboratory controlled conditions with 26°C, 70% R.H. and 14:10 photoperiod. Results indicated that there was not significant differences in LC₅₀ values, and percent growth inhibition in field collected versus the susceptible population. Dosages of 0.05 mg/ml prevented larvae from reaching the 3^rd instar and are suggested as diagnostic for discriminating susceptible and resistant strains.