During an ecological investigation of populations of the freshwater bryozoan, Cristatella mucedo Cuvier, parasitic sac-like stages of the myxozoan, Tetracapsula bryozoides Canning, Okamura et Curry, 1996 were discovered within the body cavity of some bryozoan colonies. Subsequent to their detection, data were collected on the incidence and prevalence of myxo-zoans in bryozoan populations in the Thames Valley region and on the effects of myxozoans on their hosts. Notable spatial and temporal variation in incidence and prevalence of myxozoans was documented. The production of statoblasts by bryozoans was significantly compromised by myxozoan infection although the production of larvae was not. Infection by myxozoans resulted in generalized swelling, malformation, degeneration, and slower response times in bryozoan colonies. These findings indicate that myxozoans adversely affect host fitness and may therefore influence population levels. Light microscopy revealed that multiple myxozoan sacs can be present within the continuous body cavity of bryozoan colonies and that sacs apparently undergo fission. Infective spores that develop within sacs are at least on some occasions released into the body cavity of bryozoans. Nothing is known of the subsequent stages in the life cycle of this myxozoan, including whether T. bryozoides infects another species. Spores may be released into the water column to infect new hosts or they may be introduced to new hosts when infected bryozoans are ingested.