Abstract: The blue green algae Anabaena azollae has been isolated from the leaf cavity of water fern Azolla imbricata and grown aseptically in N-free medium. It was used for the study on the effects of white, red, yellow, green and blue light illuminations on the growth, acetylene reduction activity and heteroeyst formation. The results are summarized as follows: The growth rate under white, red and yellow light was greater than under green light and the yield of the former was increased nearly two folds than the latter after 7 days culture. However, the green light enhanced heterocyst formation up to 40% than the other light illuminations. Chlorophyll content was also higher under green light. Nitrogen fixation activity was proportional to the heterocyst formation. Whatever the light quality used, nitrogen fixation activity could be increased up to several folds by adding 0.2% fructose to the culture medium. When the fructose was added to the 5 days culture, green light illumination showed the highest nitrogen fixation activity. The significant reaction of Anabaena azollae to the green light seems to be a physiological feature of the symbiont.