Department of Entomology, Louisiana State University, Baton Rouge, LA 70803 Bt corn cultivars have demonstrated significantly less infection from southwestern corn borer, Diatraea grandiosella Dyar, and sugarcane borer, D. saccharalis (F.), compared to conventional corn cultivars. Larval susceptibility of laboratory and field-collected strains of southwestern corn borer and sugarcane borer to Cry1Ab endotoxin extracted from plants of a YieldGard Bt corn hybrid, DK6970Bt, was determined using a meridic diet incorporating different concentrations of Cry1Ab toxin. High larval mortality was observed when neonates of the two species were individually reared on diets treated with Cry1Ab toxin at > 0.25 mg/kg for 8 days. Larval body weights of both species declined as Cry1Ab concentration increased. The dose-response data will be useful to determine diagnostic doses that can separate resistant and susceptible individuals.