The influence of temperature, CO2 availability, light intensity, and colour on growth of Anacystis at high (70-120 Wm 2) light intensities was investigated. In the first 2 k days rapid formation of cells with a high nitrogen content was found. During this period the nitrogen content of the cells was about 10%. When the supply of bound nitrogen was exhausted, the over-all photosynthetic mechanism switched to production of nitrogen deficient products, probably carbohydrates. Only in the first period growth could be followed by measuring optical parameters, and a production of 0.5-1 g-1 "1 dry weight per day could be obtained in single batch as well as in continuous cultures.