Transgenic corn hybrids expressing crystal protein endotoxin genes from Bacillus thuringiensis Berliner (Bt) are an increasingly popular tactic for managing the European corn borer, Ostrinia nubilalis (Hübner), in North America.  European corn borer populations are also often vulnerable to the ubiquitous entomopathogenic microsporidium Nosema pyrausta (Paillot).  Pervious work has examined the interaction of Dipel ES (Bt source), N. pyrausta, and the European corn borer.  However, Dipel ES, a commercial formulation of Bt, contains several different endotoxins in addition to Cry1Ab, the protein predominantly produced in commercially available transgenic corn hybrids for European corn borer suppression.  We examined the effect of feeding meridic diet incorporated with purified Cry1Ab on growth, development, and survival of Nosema-infected and non-Nosema-infected neonate European corn borers.  Seven concentrations of Cry1Ab in diet and diet without Cry1Ab were used.  After 10 d of feeding, weight, development, survival, and level of Nosema infection of larvae were assessed.