The morphology and physiology of two pure culture isolates of Rhizophydium patellarium Scholz have been compared with two isolates of R. capillaceum Barr. The species differ in method of zoospore discharge: R. patellarium by dissolution of part of sporangium wall and R. capillaceum by dissolution of papillae. In physiology there are differences between isolates of each species but there is no useful physiological criterion for distinguishing species. It is proposed to reduce R. collapsum Karling as a synonym of R. patellarium.