Studies on the methods of mass culture of nitrogen-fixing blue-green algae,Anabaena azotica HB 686),and A.azotica forma.alpha（HB.678）,have been carried out in the laboratory and in the open.The culture medium used（HB 105）is nitrogen-free and specially adapted to the require- ments of these algae.In the laboratory experiments,the algae were grown in shallow rectangu- lar ceramic trays under two 40-watt fluorescent lamps and supplied with CO2—enriched air. Fresh culture medium was added every day to balance loss by evaporation,and the medium was renewed after 2 to 3 days.Under these conditions the algae grew at a rate of over 100% in every 2 days,and the highest production was 834 grams of fresh algae per m2,per 20 hours illumi- nation,about 13.3 mg.dry weight/m2/20 hr.,under five 40-watt fluorescent lamps. In outdoor mass culture,these blue-green algae were grown in shallow wooden vessels of about 4 square metres in area,under direct sunlight,or in a greenhouse,without the addition of CO2.In summer,the strong noon sunlight was shaded by a bamboo screen,to prevent its harmful effects,particularly from the ultra-violet.The optimum temperature for growth was found to be between 28° and 30℃.The production of algae under favourable conditions is about a two-fold increase in 24 hours.But in most cases growth is slow at first,becoming luxuriant after a week. We found that the algae became yellowish under direct sunlight,because of the decrease in phyocyanin content,which agrees with the results of Halldal's experiments. In our experiments,the contamination of the cultures by other organisms is one of the most serious limiting factors,and we found the most harmful intruders were Chironomus larvae and Rotifera.Chironomus was controlled by the application of 3 to 5 pp m.of the insecticide “666”, while Rotifera and Protozoa were effectively reduced by the addition of 0.01% of formalin to the cultures.