In this third report of taxonomic studies on the freshwater species of Ulothrix KUtzing, the morphological and reproductive characteristics of Ulothrix implexa (KUtzing) KUtzing, U. tenuissima KUtzing and U. zonata (Weber & Mohr) KUtzing are discussed. U. implexa is characterised by the lack of gametogenesis whereas the absence of asexual reproduction, occasionally occurring in culture, is accompanied by a very frequent appearance of branched rhizoid-like outgrowths at irregular intervals in the filaments. The chloroplast morphology may show an affinity to the genus Klebsormidium Silvia, Mattox & Blackwell. U. tenuissima shows both asexual and sexual reproduction. The occurrence of curled filaments containing gametangia and the occasionally pointed zoospores and gametes are characteristic for this species. In a certain growth stage of the chloroplast some morphological resemblance to the genus Urospora Areschoug (Hormiscia Fries) is present. U. zonata shows an identical life cycle as U. tenuissima. It is proved that numerous Ulothrix species which are described only are growth habits of U. zonata. Both under long- and short-day conditions U. implexa only shows reproduction by means of zoospores, whereas the other species, referred to in this paper, show asexual stages under a short-day light regime, and gametogenesis under a long-day photoperiod; sporophytes are observed to be fertile under short-day conditions. The manner of occurrence of the species in nature agrees with the behaviour of the algae under different photoperiods in culture.