Abstract: Seven free-living strains of RhiRiobium, isolated from the nodules of four species of tropical leguminous plants, have been studied for their adility of acetylene reduction. Four strains of Rhtizobium, viz. 8638L 8638M and 8638S, Albizia falcataria, and strain PS Psophocarpus tetrago-nolobus produced more than trace amounts of acetylene reduction activity. Among them, Rhizobium sp. strains 8638L and PS yield higher nitrogenase activity. A medium containing a low-level source of nitrogen (Glutamine), a conbination of two carbohydrate (Arabinose and Succinate) and low oxygen tension appears necessary for the expression of high level of nitrogenase activity.The results showed that the optimal concentrations of oxygen for both strains 8638L and PS, were 1%, while the concentrations of Glutamine for the strains 8638L and PS were 2 mMol/L or mMol/L, respectively. The nitrogenase activity of both strains cultured on the medium being hi-hger under pH 5.6-6.0.