Bollgard II® cotton expresses the ä-endotoxins Cry1Ac and Cry2Ab of Bacillus thuringiensis Berliner, controls larvae of Helicoverpa zea (Boddie), and will probably be used in the near future in Mexican agriculture.  However, the initial susceptibility to this toxin in populations of H. zea is unknown.  The objective of the present study was to determine the susceptibility to the ä-endotoxin Cry2Ab in neonate larvae of five populations (one susceptible and four field populations) of H. zea.  The observed mortality levels were low (<43%) in the evaluated doses (=5 ìg mL-1).  The estimated concentration necessary to prevent 50% of the exposed larvae from reaching the third instar or inhibition of development of 50% of treated larvae (ID50) varied from 0.017 to 0.086 ìg mL-1, while the ID95 varied from 1.06 to 1.93 ìg mL-1.  The concentration that reduced or inhibited 50% of the weight of treated larvae (IP50) varied from 0.013 to 0.025 ìg mL?1, and the IP95 from 0.5 to 1.48 ìg mL-1.  The obtained toxicity values to ä-endotoxin Cry2Ab in larvae of H. zea are considered as initial response or reference point, given that this toxin is still not used in the field and will serve to estimate the proportion of change over time once Bolgard II® is used in México.