Abstract: The effects of Anabaena azollae on the heat tolerance of Azolla were conducted.Under optimum temperature regine anabaena-free Azolla grown in nitrogen-containing medium produced less biomass compared with Azolla-Anabaena association.It implied that Anabnena in the association was not only supply nitrogen to Azolla but also provied something else which was required for Azolla growth.At 37℃/29℃ (day/night) Azolla filiculoides grew poor than the corresponding Anabaena-tree Azolla grown in nitrogen-containing medium while Azolla filiculoides with the Anabaena from Azolla microfhylla (Af 135),a newly reconstituting Azolla,was able to grow much better than the Azolla with homologous Anabaena.However,Azolla microfhylla with the Anabaena from Azolla filiculoides grew and fixed nitrogen less than original one.the growth of Azlolla microfhylla was much better than Azolla filicwifos with the Anabaena from Azolla microfhylla.Therefore,the tolerance of Azolla to high temperature is determined by both host-fern and symbiont-Anabaena.