Infection by the pathogenic fungus Entomophthora muscae was monitored in muscoid fly populations on four caged-layer poultry facilities in southern California. Adult flies were captured every 1 to 2 weeks with sweep nets and held for 7 days in the laboratory to assess incidence of infection over a 2-year period. Patent Musca domestica and Ophyra aenescens infections were found throughout the year. Average prevalence of infection in M. domestica was highest (45 percent) in fall and lowest (replacecodegt 1 percent) in the hot summer months. Infection in O. aenescens at times approached 100 percent, with three peaks of E. muscae activity (March, June, November) coincident with peak population densities. Infections in Fannia canicularis and F. femoralis were evident primarily in late spring. Peak average infections in F. canicularis (45 percent) lagged behind peak adult population density by 3 weeks in 1983. Infections in Fannia spp. were rare in 1984 due to low rainfall and subsequent low adult densities. Infection prevalence was higher in male than female hosts, but was probably influenced by the shorter incubation period in males and the short holding period. Flies were not infected with E. muscae as larvae or while emerging from manure. Cool fall weather stimulated morning M. domestica aggregation on the south walls of the primary poultry house study site, where cadavers killed by E. muscae were most common. Flies likely were infected primarily by morning exposure to secondary conidia expelled from these structure surfaces. Possible management techniques to optimize E. muscae activity are discussed.