The present study, based on rearing experiments and herbarium investigation, deals with the taxonomy of three freshwater Ulothrix species; Ulothrix subtilis Kutzing, U. tenerrima Kutzing, and U. albicans Kiitzing. Laboratory cultures have to be used for unequivocal determination. Of special importance are developmental features like the shape of the basal cell, and life-history features like the limited variation in number of zoospores and gametes. When many filaments are present and when culturing is impossible, the limited variation of the cell diameter of vegetative cells, zoosporangia and gametangia can also be used, with care. The behaviour of the species under different photoperiods is correlated with the way the algae appear in nature in winter and summer time.