In 1997 and 1998, Cry9C susceptibility baselines were established for field-collected populations of European corn borer, Ostrinia nubilalis (Hübner), and southwestern corn borer, Diatraea grandiosella Dyar. Bioassay of neonate European corn borer larvae of 16 colonies collected from the midwestern United States indicated LC₅₀ values ranging from 13.2 to 65.1 ng of Cry9C protein per square centimeter. Neonate European corn borer LC₉₀ values ranged from 46.5 to 214 ng/cm². Neonate larvae of three colonies of southwestern corn borer collected from the southern and southwestern United States exhibited LC₅₀ values from 16.9 to 39.9 ng of Cry9C protein per square centimeter. Southwestern corn borer neonate LC₉₀ confidence limit values ranged from 40.3 to 157 ng of Cry9C protein per centimeter. The most sensitive southwestern corn borer colony was collected from the Mississippi delta exhibiting an LC₅₀ value of 22.6 ng of Cry9C per cm² and also displayed the widest LC₉₀ confidence limits of 40.3-94.8 ng of Cry9C per cm². Geographic baseline susceptibility data establishes the natural genetic variation and provides the foundation for future testing of insect populations exposed to increased use of Bacillus thuringiensis-based crops. Insect resistance management and stewardship of Cry9C will rely upon baseline data for the validation of discriminating dose assays for European corn borer and southwestern corn borer.