Abstract: One hundred and fifty one nitrogen fixing strains were isolated from twenty one Poplar clones and half families root systems and rhizospheres in different sites. Eight strains, nitrogenase activities were higher than 100(nmol C 2H 4/bottle·h). Different Poplar clones and half families rhizospheric nitrogen fixing bacteria, quantity、speciese and distribution were different. Five strains(No.125、3、71、74、134) with the highest nitrogenase activity were tentatively identified, No.3 and 71 strain both belong to Beijerinckia ,No.74 strain belongs to Derxia . The taxonomic position of No.125 and 134 strains remained unidentified. These strains isolated from Poplar rhizosphere are essentially identical with some nitrogen fixing bacteria at grasses and grain crops root systems.