The nitrogen fixation,activities in soil cores of rice rhizosphere were measured by the acetylene-reduction method.As calculated according to the theoretical value of the ratio of acetylene reduced to N fined (3:1),the amount of nitrogen fined in paddy field of early rice ranged from 200-226 g per mu for 66 days and that of late rice from 301-398 g per mu for 72 days respectively,in which about 80-90% of N₂ was fined from the heading to ripening stages for early rice and 40-52% of N₂ was fixed during the same growing period for late rice.The results of experiments showed that the nitrogen fixation activity in rice rhizosphere varied with different parts of root.The highest activity was found at the basal part of stem connect with root system buried in soil about 2.5 cm,the lower was at the roots below basal stem about 3 cm in soil and the lowest was at the roots near root tip.Among the heterotraphic bacteria associated with rice plant routs,nitrogen-fixing organisms was dominant,with a few Clostridium butyricum.