Periodical cicada nymphs (Homoptera: Cicadidae:Magicicadaspp.) that were collected as they emerged from the ground, isolated, and allowed to eclose in individual containers sometimes developed infective conidia of the fungusMassospora cicadinaPeck, but never resting spores. The time required for the cicada's abdominal sclerites to fall off (i.e., for the animal to become highly infective to others) and the proportion of nymphs developing the infection did not differ between the sexes. By contrast, in an adult population, the prevalence of secondary infections producing resting spores was much higher in males. The proportion of individuals infected as nymphs was positively correlated with the time since the habitat was last disturbed, and negatively correlated with the number of cicadas emerging per m2. Cicadas in one of the six study sites had been studied in the prior emergence—17 years earlier—which provided the opportunity to determine what changes had occurred in cicada density and the proportion infected by the fungus. Density had dropped by one-half, and the proportion of infected individuals generating resting spores had increased by 9-fold. These data suggest possibilities for use of the fungus as a biological control agent, although the resting spores appear to be highly vulnerable to environmental disturbance.