Japanese beetle,Popillia japonicaNewman, population densities and prevalence of the pathogensOvavesicula popilliaeAndreadis&Hanula andBacillus popilliaeDutky are reported from three locations in Connecticut over a 3-yr period. Populations decreased to very low levels (50%). Conversely, prevalence ofB. popilliaewas<30% throughout most of the study.Ovavesicula popilliaereduced fecundity 50% in heavily infected females in the laboratory. In addition, maximum prevalence ofO. popilliaein adults, collected in attractant-baited traps, peaked several weeks after peak adult emergence in the field, suggesting thatO. popilliaeslows development. However, no differences were detected in the weight or development of field-infected third instars reared in the laboratory. It is suggested that the effect ofO. popilliaeon its host is modified by environmental stress.