Fusarium ear rot of corn (Zea mays L.) is associated with feeding damage from the European corn borer (ECB), Ostrinia nubilalis Hübner, and the corn earworm (CEW), Helicoverpa zea Boddie. Specific transformation events encoding for Cry1Ab protein from Bacillus thuringiensis Berliner (Bt) may reduce Fusarium ear rot and fumonisin concentration in grain by minimizing damage from certain insects. The objective of this study was to determine if effects from Cry1Ab protein in kernels and silks on fumonisin concentration in grain vary depending on the genotype of the hybrid or the predominant insect species. Four Bt corn hybrids and their corresponding nontransgenic, near-isogenic hybrids were compared for ear rot severity and fumonisin concentration in grain in four environments. Treatments included inoculation with F. verticillioides (Sacc.) Nirenb. (Syn = F. moniliforme J. Sheld.) and F. proliferatum (Matsushima) Nirenb., infestation with ECB larvae, infestation with CEW larvae, and controls. Cry1Ab protein from the Mon810 transformation event was associated with reduced ear rot severity when hybrids were not inoculated with Fusarium spp., regardless of whether hybrids were infested or not infested with insects. Cry1Ab protein was associated with reduced fumonisin concentration in grain when ECB was the predominant insect, but not when CEW was the predominant insect. Cry1Ab protein was not associated with reduced fumonisin concentration in grain for the most resistant hybrid pair in this study. Results suggest that Bt hybrids can reduce fumonisin concentration in grain during seasons when ECB is favored, but not during seasons when CEW is favored. Hybrid genotype was an important factor in reducing fumonisin concentration in grain. Abbreviations: Bt, Bacillus thuringiensis ? CEW, corn earworm ? ECB, European corn borer ? ELISA, enzyme linked immunosorbent assay ? FDA, Food and Drug Administration ? FB1, fumonisin B1 ? FB2, fumonisin B2 ? FB3, fumonisin B3 ? HPLC, high-performance liquid chromatography ? PBS, phosphate buffered saline ? PDA, Potato dextrose agar ? R2, blister developmental stage of corn ? R4, dough developmental stage of corn ? USDA-ARS-NCAUR, United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research ? V8, developmental stage of corn when eight leaves with collars are visible ? VT, tasseling developmental stage of corn