The use of different commercial Bacillus thuringiensis (Bt) products in the Bajio guanajuatense area in Mexico began 12 yr ago, and resistance to Bt in this area has been reported for Plutella xylostella (L.) The current study provides a baseline response and resistance potential to Bt in field and laboratory strains of Bajio Trichoplusia ni (Hübner). Differences in susceptibility to Bt among T. ni populations were observed. T. ni neonates collected in Romita, Guanajuato, were more susceptible to Bt than those collected in Salvatierra or San Luis de la Paz, Guanajuato. After five generations of exposure to XenTari in the laboratory, decreased susceptibility was found only in the Salvatierra insects, with an LC₅₀ that was 2.1-fold greater than that of a Mexican laboratory strain. The XenTari-selected San Luis de la Paz strain was from 16- to 87-fold more resistant to Cry1A protoxins than U.S. (US) and Mexican laboratory strains. Although Cry1Ab is not a component of XenTari, this strain also was significantly less susceptible to Cry1Ab toxin compared with a US strain, with a resistance ratio of 40.4. The larval weights and lengths, pupal lengths, and percentage of pupation were significantly lower for the Salvatierra strain than for all other strains. The relationship of T. ni susceptibilities to Bt Cry toxins and protoxins after several generations of exposure to XenTari and its similarity to P. xylostella behavior.