Abstract: The N2- fixation activities of Phyllestachys pubescens and Ph.meyeri's roots have been determined by N2- backfilling and enrichment culture method respectively.In N2- backfilling method,two kinds of bamboo can reach 9.9 and 2.1 nmol C2H4 per gram of dry roots per hour respectively.When tested by enrichment culture method,the activities of over 40% of the roots are within the range of 20～100 nmoI C2H4 per gram of fresh roots per hour.The amount of N2- fixation bacterium in Ph.meyeri's rhizosphere soil on the surface and in the roots have been determined by the method of acetylene reduction-MPN,the results of wbich are 3.4 ×105 bacterium per gram in dry soil and 2.2×105 (surface) and 3.0×104(roots) bacterium per gram of dry roots.The diazotroph of Ph.meyeri's rhizosphere have been isolated by streaking,three strainswith rather high N2-fixation activities have been isolated and identified,which belong to Enterobacteriaceae with number of G-4、G-7 and G-12 respectively.Further studies show that G-4 belong to Klebsiella pneumoniae;G-7 and G-12 are of the same kind of bacterium,but their genus has not been determined in this study.It shows that these two associative diazotrophs have the highest N2- fixation enzyme activities after being cultivated in the improved Dobereiner medium(without nitrogen) for 18～24 hours and they can suit wide pH,from 4.5～9.2,but for the bacteria G-7,the optimum pH value is around 7.5 and for G-4,pH 6.5 seems to be the best.When the incubation temperature is about 30℃,their highest nitrogen fixing activities can be measured,and many kinds of carbon sources can be used by these associative diazotrophs.