Abstract:The symbiotic relationships between Azolla and Anabaena azollae were studied by means of autoradiography after the Azolla was administered by 3H-thymidine, 3H-uridine, 3H-leucine and 3H-glucose. The experimental results showed that the four labeled compounds mentioned above were transfered from Azolla to Anabaena azollae through cavity hairs. This indicated that there was a transfer way of substances from fern to algae. It is suggested that the symbiotic relationship between Azolla and Anabaena azollae is more complicated than we have known up to now. The fern not only get the NH3 which was formed by symbiont-blue alga, but also supplied some nitrogen-containing substances, such as amino acids (or proteins), ribonucleotides for symbiotic algae. Although the symbiont still retained photosynthetic ability, the ability of nitrogen fixation might be developed and the photosynthetic autotrophic ability might be dropped gradually in the long symbiotic life and the Anabaena azolla needed take a portion of substances from the Azolla as replenishment.