Five species of Cyathodium occur in the Neotropics. The species grow in shaded habitats that range from unstable river banks to cement pots and road ditches. Cyathodium species are r-selected species. Two species are dioicous and three are monoicous. Vegetative reproduction by tubers is restricted to dioicous species that also produce abundant sporophytes. All monoicous species produce abundant sporophytes. Sporeling patterns of C. cavernarum and C. foetidissimum grown under culture conditions include an initial filamentous phase followed by the apical development of thalli. Cyathodium spruceanum produces a multiseriate bud-like protonema from which the thallus develops. Twelve gametophytic and two sporophytic characters were selected to discriminate between species in vegetative and fertile states. Nucleotide sequence variation in the nuclear ribosomal DNA region, ITS1-5.8S rRNA-ITS2 was analyzed for four species. Each species is genetically and morphologically distinctive. The largest genetic differences were found between C. foetidissimum and C. spruceanum. Sequences for C. bischlerianum failed and were not included in the analysis. Samples of C. cavernarum and C. spruceanum from nearby geographical areas were shown to be genetically more closely related than to those of geographical distant areas.