In this fourth report on the taxonomy of Ulothrix Kützing a new classification of the marine and brackish-water species in western Europe is proposed. Comparative studies on field collections, uni-algal cultures, herbarium collections and sections prepared for electron microscopy lead to the recognition of three marine species, viz. Ulothrix speciosa (Carmichael ex Harvey in Hooker) Kützing, U. flacca (Dillwyn) Thuret in Le Jolis, and U. palusalsa Lokhorst (nov. nom.), and two brackish-water species, viz. U. implexa (Kützing) Kützing and U. subflaccida Wille. The vegetative anatomy, the life history, the fine structure of the vegetative thallus and the distributional pattern in nature are amply discussed. Salient, reliable characters proved to be, e.g., the nature and construction of the cell wall, the texture of the cell wall’s surface, the fine structure of the pyrenoid, the developmental stages of germinating zoospores, the coalescense of filaments, the shape of the gametangial filament, and the limited variation of the number of zoospores and gametes. A brief discussion is given of the ecological status of the individual species. In addition there is a brief comment on the taxonomic affinity of Ulothrix with the morphologically related genus Urospora Fries and on the phyletic relationship of Ulothrix with the progenitors of the higher land plants. The reproductive behaviour of the species under different photo periods in culture appeared to be correlated with the seasonal periodicity expressed by the algae in nature.