Abstract:Five endophytic diazotroph strains, including POJy11, POG12, POGy22, POGy612 and POG4 from rhizosphere and stalks of Paspalum orbiculare, were studied in acetylene reduction activity (ARA). ARA was maximal at pH 6.0-10.0 in POGy612 and POG4 strains and did at pH 6.0-8.0 in POJy11, POG12 and POGy22 strains. The maximum activity occurred at 35℃ in POGy22 strain and did at 30℃ in the other strains. The activity in those strains remarkably declined as NH4+ or NaCl concentration increased. ARA was mostly inhibited by NH4+ in POGy22 strain, and lost activity over 1.0 m mol /L NH4+. ARA showed a peak of activity at 2.5 mmol/L NH4+ in POGy612 strain and did at 0.5％NaCl in POJy11 and POGy22 strains.