The microsporidan Pleistophora sp. is a common parasite of Crangon franciscorum, C. nigricauda, and C. stylirostris in the vicinity of Yaquina Bay, Oregon. Characteristics of the parasite are described. Skeletal muscle was the only host tissue infected. The seasonal prevalence and intensity of the parasite in crangonids are described, based on examination of 1,556 C. franciscorum, 3,877 C. nigricauda, and 1,674 C. stylirostris collected at monthly intervals from June, 1975, through June, 1976. Prevalence in C. franciscorum and C. stylirostris increased through the fall and reached winter peaks of 30.3% and 41.0% respectively, then decreased in the spring. Prevalence in C. nigricauda remained below 8% through the year. Intensity increased with size of the shrimp in the three species. Infection experiments and field observations indicate that only very young shrimp are susceptible to infection during a relatively short period during the summer months. Following initial exposure, the infection spread within the host, indicating repeated schizogonic cycles. Parasitic castration was indicated by the absence of gravid infected female shrimp and was confirmed by histological examination. Ovaries of infected shrimp did not develop beyond a very early stage. A shift in sex ratio toward females in infected shrimp also indicates that the parasite may influence sex determination. Shrimp showed little cellular response to infection. Only rarely in heavily infected shrimp was encapsulation of the parasite cysts observed, and necrotic tissue was occasionally observed. Infected shrimp succumbed before uninfected shrimp under low oxygen stress. The collection of unusually large infected shrimp indicates that these shrimp either experienced accelerated growth or lived longer than uninfected shrimp. / Graduation date: 1977