Abstract:The cycas distributed in the dry-hot inshajiang river valley in the south-subtropical region of China,infected by the cyanobacteria,form especial coral-like nodule clusters with multiply branches. The nodules are most numerous within 10cm below the soil surface. The living nodules of current seedlings amounted to 8 g per plant and those of over 100 year plants averaged 370 g. The nitrogen fixation activity is usually from 1.8 to 11.1 μmol/g·f·w·h-1 in the autumn, and is obviously influenced by light and soil moisture, Daily variation of N2-fixing activity has shown that it is mhcu higher in daytime than at night. Nitrogen fixation amount for the cycas varied from 0.64 to 18.68 mg/plant·h-1.This suggests that it may play a certain role in the nitrogen cycle of