Abstract: Heat tolerance was determined for Anabaena azollae. The decrease in the variable fluorescence (F_v) yield at 680 nm after heating filaments of A. azollae at 37℃, 45℃and 50℃ each for 5 min. was found as compared with that at 25℃. The ratio of F_v to instantaneous fluorescence (F_o)was 0.35 at 25℃ and the decreases following heating at 37℃and 45℃ were found. And these ratios were 0.27 and 0.11, respectively, As compared with that at 25℃, the relative heat tolerances were 77.2% and 31.5%, respectively. The results showed that the filaments of A. azollae can stand with the temperature stress lower than 37℃ typical observed in the summer.