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Transovarial transmission was found to be the major transmission route of a microsporidian infecting Gammarus duebeni. The parasite was found in 31% of females in a field population but was never found in males. The infection was localized in the ovary and, in laboratory investigations, had no discernible effect on the growth rate, survival, or reproductive output of infected hosts. More than 90% of the offspring of an infected mother became female. Parasite-induced feminization of the host is a major factor in the maintenance of the microsporidian in the host population.