Nosema pyrausta(Paillot) detrimentally affected the development of its host,Ostrinia nubilalis(Hübner). Larvae exposed to the microsporidium during the first two stadia formed abnormal pupae or emerged as abnormal adults. Infections of later instars reduced average longevity of resultant adult females by at least 2 d and fecundity by at least 50%. Eggs from infected adults were contaminated with the microsporidium. The prevalence of transovarial-transovum infections, determined by the presence of spores in eggs or in emerging larvae, varied with the spore concentrations to which the parent females were exposed and with the time (within the oviposition period) that the eggs were laid. Per os infection ofO. nubilalislarvae withN. pyraustais important in maintaining this microsporidium in a population ofO. nubilalisas well as reducing the vitality of the population.