In this report on the taxonomic studies on the genus Urospora Areschoug in Western-Europe, a reclassification of the species is proposed. Comparative studies were made on natural collections, uni-algal cultures, herbarium collections and sections prepared for electron microscopy. These studies confirm the recognition of four species, viz. Urospora wormskioldii (Mertens in Homemann) Rosenvinge, U. bangioides (Harvey) Holmes & Batters, U. penicilliformis (Roth) Areschoug and U. neglecta (Kornmann) nov, comb. The morphology of the gametophytic filament and the uni-celled sporophytic ( (Codiolumi-)phase, the life-history, the nomenclature and historical aspects, and details of the ultrastructure of the vegetative thallus and the zoospores (in particular the flagellar apparatus) are discussed. Reliable, diagnostic characters for these species are shown to be the degree of rhizoidal development in the most basal cell and its neighbouring cells, the shape of mature vegetative cells in the filaments and their cell diameter, the dimensions of zoospores and gametes, and the presence of dwarf plants in the life cycle. The shape and size of the developing CW/o/um-plants are shown to vary with external conditions. The Codiolum-plants in culture are clearly shorter than their natural counterparts. Moreover, differently shaped Codiolum-plants, collected from various natural populations, proved to belong to the life-history of the same Urospora species. A brief discussion is given to the taxonomic status of the genus Codiolum Braun. No species-distinguishing ultrastructural features were found among these four species. The micro-anatomy of the pyrenoid and the unique flagellar apparatus proved to be similar for all Urospora species. At the fine structural level, Urospora forms a clear-cut taxonomic unit.