Cercariae of Proterometra dickermani (Digenea: Azygiidae) grow to mature adults in tissues of the snail host Goniobasis livescens. A total of 1,241 G. livescens was collected during the summers of 1980-1982, 1984, and 1987-1989 from the Ocqueoc River in northern Michigan. Infected snails usually contained both cercariae and adults, but only adults emerged from snails in the laboratory. Behavior and morphology of naturally emerged adults and adults obtained by dissection were similar. Observations in the laboratory showed emergent adults shed hatchable eggs, and it is assumed that other snails can be infected by ingesting those eggs. Prevalence of infected snails decreased progressively from 85% in 1980 to 4% in 1989. The gradual disappearance of both P. dickermani and its snail host from the Ocqueoc River may be related to habitat destruction from soil erosion in the watershed and the application of chemicals to control sea lampreys. Because P. dickermani is now known to exist only in the Ocqueoc River, it should be considered an endangered species.