Abstract: A method for obtaining axenic culture of blue-green algae was developed. A pure culture of Anabaena azollae was obtained with this method. A. azollae could grow heterotrophically in the dark. Fructose, glucose and sucrose supported the hcterotrophic growth of A. azollae. The culture adapted to photoautotrophic growth grew better in the dark when NaNO3 was used as nitrogen source, but the culture adapted to heterotrophic growth grew better when N2 in air was used as nitrogen source. Stirring stimulated heterotrophic growth of A. azollae. The concentration of chlorophyll a of the culture grown in the dark for half year decreased to 1/3—1/4 of that in the light. When A. azollae grew at 5500 lux, exogenous fructose and glucose still stimulated growth and increased nitrogen fixation.