The microsporidium Pseudoloma neurophilia was initially reported to infect the central nervous system of zebrafish causing lordosis and eventually death. Subsequently, muscle tissue infections were also identified. To understand the infection process, development, and ultrastructural pathology of this microsporidium, larval and adult zebrafish were fed P. neurophilia spores. Spores were detected in the larval fish digestive tract 3-h postexposure (PE). By 4.5-d PE, developing parasite stages were identified in muscle tissue. Wet preparations of larvae collected at 8-d PE showed aggregates of spores in the spinal cord adjacent to the notochord. All parasite stages, including spores, were present in the musculature of larval fish 8-d PE. Adult zebrafish sacrificed 45-d PE had fully developed infections in nerves. Ultrastructural study of the developmental cycle of P. neurophilia revealed that proliferative stages undergo karyokinesis, producing tetranucleate stages that then divide into uninucleate cells. The plasmalemma of proliferative cells has a previously unreported glycocalyx-like coat that interfaces with the host cell cytoplasm. Sporogonic stages form sporophorous vacuoles (SPOV) derived from the plasmalemmal dense surface coat, which "blisters" off sporonts. Uninucleate sporoblasts and spores develop in the SPOV. The developmental cycle is identical in both nerve and muscle. The SPOV surface is relatively thick and is the outermost parasite surface entity; thus, xenomas are not formed. Based on the new information provided by this study, the taxonomic description of the genus Pseudoloma and its type species, P. neurophilia, is modified and its life cycle described.