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Susceptibility to CrylAb and CrylAc toxins from Bacillus thuringiensis was determined for 11 populations of neonate European corn borer, Ostrinia nubilalis (Hiibner), from the United States and 1 from northern Italy. Corn borer larvae were exposed to artificial diet treated with increasing B. thuringiensis concentrations, and mortality and growth inhibition were evaluated after 7 d. The range of variation in B. thuringiensis susceptibility indicated by growth inhibition was very similar to that indicated by mortality. Although interpopulation variation in susceptibility to both proteins was observed, the magnitude of the differences was small (54-fold) and comparable to the variability observed among generations within a particular population (53-fold). Additionally, there was no indication that B. thuringiensis susceptibility was influenced by pheromone race, voltine ecotype, or geographic location. These results suggest that the observed susceptibility differences reflect natural variation in B. thuringiemis susceptibility among corn borer populations rather than variation caused by prior exposure to selection pressures. Therefore, European corn borers apparently are susceptible to B. thuringiensis toxins among populations across most of their geographic range.