Field experiments were conducted in 1994, 1995 and 1996 to evaluate the incidence and severity of Fusarium ear rot and the incidence of symptomless Fusarium infection in kernels of maize hybrids genetically engineered with Bacillus thuringiensis genes encoding for the delta -endotoxin CryIA(b). Treatments included manual infestation with European corn borer (Ostrinia nubilalis) larvae and insecticide applications to limit O. nubilalis activity to specific maize growth stages or mimic standard O. nubilalis control practices. Fusarium symptoms and infection were affected by the specific cryIA(b) transformation used in each hybrid that determines tissue-specific expression of CryIA(b). In hybrids expressing CryIA(b) in kernels, incidence and severity of Fusarium ear rot and incidence of symptomless kernel infection were reduced compared with near-isogenic hybrids lacking cryIA(b) genes. In plants that were manually infested with O. nubilalis, ear rot incidence was reduced by 87, 58 and 68%; severity was reduced by 96, 54 and 64%; and incidence of kernel infection by Fusarium species was reduced by 17, 38 and 38% in 1994, 1995 and 1996, respectively. Results were similar in treatments that were not manually infested, but differences between transgenic and nontransgenic hybrids were smaller. Most kernel infection was due to F. moniliforme [Gibberella fujikuroi], F. proliferatum and F. subglutinans [G. fujikuroi var. subglutinans] collectively, and it was within this group that transgenic hybrids exhibited reduced infection. Expression of CryIA(b) in plant tissues other than kernels did not consistently affect Fusarium symptoms or infection. Disease incidence was positively correlated with O. nubilalis damage to kernels. Insecticide applications also reduced Fusarium symptoms and infection when applied to nontransgenic plants.