ABSTRACTThe morphology and development ofByssothecium flumineum, a new loculoascomycete in the Dacampiaceae from submerged wood, was studied. This species is characterized by medium-sized, ovoid to obpyriform ascomata with a relatively wide peridium. Hamathecium consists of gel-coated cellular pseudoparaphyses interspersed among abundant basal and slightly lateral asci. Ascospores are 3-septate, brown with slightly lighter end cells and verruculose.Byssothecium flumineumoccurs frequently on woody debris in midwestern streams and decomposes wood by soft rot activity.