Nosema pyrausta (Paillot) detrimentally affects the development of its host the European corn borer, Ostrinia nubilalis (Hubner). Larvae exposed during the first two instars to 100 to 800 spores per mm('2) diet surface for 48 hours failed to pupate or emerge as morphologically normal adults. Infections of the later instars reduced the female adult longevity by 2 days and fecundity by at least 50%. The overall rate of transovarial infection increased by 15% when the spore concentrations were increased by a factor of eight. Also, the infection in the eggs increased from 37 to 54% from day 1 to day 7 of the ovipositional period. The pattern of infection in the eggs and subsequent larvae was similar, however, it was 18% higher in the larvae;Histological studies show that all parts of the reproductive organs, regardless of the stage of development, were susceptible to infection by N. pyrausta. In immature ovaries, the microsporidium infected the epithelial cells, stroma and germ cells. This infection subsequently spread to follicle cells and finally to the yolk;Nosema pyrausta detrimentally affected Trichogramma nubilale Ertle and Davis, decreasing the number of parasitoid adults emerging from infected hosts by 36% and the fecundity of these adults by 41%. The microsporidian spores, which were confined to the gut lumen in the larval stage, caused extensive infection in the gut epithelial, connective, neural and muscle tissues of the pupal and the adult stages;Chrysoperla carnea (Stephens) larvae fed N. pyrausta-infected eggs consumed the same number of prey compared to those of chrysopids fed uninfected eggs. They developed normally and were able to produce adults with unimpaired fecundity, fertility and longevity. The spores remained in the midgut throughout the larval feeding period and were eliminated in meconium during imaginal eclosion. At this time the spores had been within C. carnea for at least 7 days however, the spores maintained virulence in bioassays against European corn borer larvae.