SUMMARY1.Eight species and one form of the genusAphanomyces, from a number of collections, have been studied critically to determine those characteristics which serve to demarcate most clearly the various species.2.At the present time the oöspore characters appear to offer the most reliable criteria for the separation of the species studied.3.Aphanomyces helicoidesMinden is reported for the first time from America.4.A species ofAphanomyces, A. Sparrowii, parasitic onNitella, is described.5.A species ofAphanomycesmost closely related toA. parasiticus, but differing in its saprophytic nature and smaller dimensions, is described asA. amphigynus.6.A form ofA. laeviswhich differs from the species in the much smaller dimensions of oögonia and oöspores and in the diverticulate nature of the antheridial cluster is discussed.In closing, the writer takes great pleasure in expressing his deep appreciation to Dr. F. K. Sparrow, Jr., who has given generously of both time and materials and under whose direction this study was carried out. He is also indebted to Mrs. Lucy Olson, who prepared the Latin diagnoses contained in this paper, and to the Department of Botany of the University of Michigan, who extended the privileges of their laboratories.