Abstract: Three species of nitrogen-fixing blue-green algae, Anabaena azotica （HB 686）,Anabaena cylindrica and Anabaena 7120, were studied with reference to nitrogenase activity in intactcells as well as in cell-free extracts. Although the nitrogenase activity of intact cells of HB 686（51.9mmC2H4/O. D./30min.） was higher than A. 7120 （16.4mmC2H4/O. D./30min.）, itsactivity in the crude extract was the lowest. This may be due to the abundance of mucilagesin the algal mass. The effects of Mn++ and Fe++ on the nitrogenase activity of these algae weretudied also, and the reaction kinetics with different enzyme concentrations were estimated. Theresults obtained point out that in A. cylindrica there is a lack of activating cofactor, which hasbeen demonstrated in Rhodospirillum rubrum. Besides, the filaments of Anabaena 7120 weretreated with toluol-ethanol and their N2-fixing activity in situ was estimated in an attempt tofind out the mechanism of protection from oxygen.