During a study of natural control agents affecting the European corn borer,Ostrinia nubilalis(Hbn.), in Massachusetts, we determined the presence ofNosema pyraustaePaillot in the borer populations. The latter is a microsporidian parasite of the corn borer which reduces larval growth and survival rates and adult life span (Kramer 1959b, Zimmack et al. 1954). Infected females lay fewer egg masses containing fewer eggs, about half of the latter being infected transovarially (Kramer 1959b, Zimmack and Brindley 1957). Kramer (1959a) reported that this disease interacts with temperature extremes to cause mortality in host populations in the field. The morphology and life cycle ofN. pyraustaehave been studied (Hall 1952, Kramer 1959c).Raun et al. (1960) described a quantitative method to determine incidence and intensity of infection and gave such data from corn borer samples taken from 8 different states. Among the states represented were Pennsylvania and Delaware, the only eastern states from which this disease has been reported.