Transgenic corn, Zea mays L., hybrids expressing crystal protein endotoxin genes from Bacillus thuringiensis Berliner are an increasingly popular tactic for managing the European corn borer, Ostrinia nubilalis (Hübner), in North America. O. nubilalis populations also are often vulnerable to the ubiquitous entomopathogenic microsporidium Nosema pyrausta (Paillot). We examined the effect of feeding meridic diet incorporated with purified Cry1Ab on growth, development, and survival of Nosema-infected and uninfected neonate O. nubilalis. Infected larvae developed more slowly than uninfected larvae. Increasing the concentration of Cry1Ab in diet reduced larval development, and this effect was amplified by microsporidiosis. Infected larvae weighed significantly less than uninfected larvae. The relationship among Nosema infection, Cry1Ab concentration, and larval weight was fitted to an exponential function. The LC₅₀ of infected larvae was one-third that of uninfected larvae, indicating that infected larvae are more vulnerable to toxin. This work has implications for resistance management of O. nubilalis and demonstrates that it is important to determine whether N. pyrausta is present when testing susceptibility of larvae to transgenic corn hybrids.