The susceptibility among field collections of European Corn Borer, Ostrinia nubilalis (Hübner), from across the corn belt has been assessed since 1996 to determine if exposure to transgenic Bt corn has caused a detectable change in susceptibility. In 2001, a field collection of European corn borer (ECB), from Kandiyohi County, Minnesota exhibited less than 99% mortality at a diagnostic concentration derived from baseline susceptibility studies. A colony was developed from the survivors for further characterizations of Cry1Ab resistance. We used one selected strain and one susceptible strain to quantify the level of resistance and perform bioassays to characterize the inheritance pattern. We used these two colonies to establish reciprocal crosses between RR and SS parents, and backcross between RS progeny and RR parents. The F1 neonates were then bioassayed and compared with both parental populations. These studies will determine whether the resistance is autosomal or sex-linked and provide preliminary assessment of the number of genes involved in resistance.