Based on experiences of Grote (1977) with Spirogyra majuscula. methods are described to induce the mating process and the formation of spores in several Spirogyra species. Nitrogen depletion and light appear to be the key factors for the induction of spores in Spirogyra species. In the present study experiments in batch cultures were carried out with clonal material growing in defined medium or water from field localities. 31 Spirogyra species could be induced to spore formation which is about 40% of the total number of species as yet recorded in The Netherlands. The amount of spores produced showed considerable inter- and intraspecific variation. The influence of environmental factors like temperature and light intensity on the spore production was also studied in some species. For Spirogyra hassallii the combined effects of light intensity and temperature were studied with the aid of a temperature/light gradient plate. Ripened spores were formed under a broad range of temperature and light conditions. The role of N-depletion and its ecological implications are discussed.