In biological nitrogen fixing (BNF), symbiosis Rhizobium infects the root of leguminous plants. The potential of this symbiosis is its specificity for Rhizobium strains. To assess the success of BNF in the agricultural area of Western Azarbaijan, Iran, the nitrogen fixation potential of the Iranian native inoculators Rhizobium strains isolated from root nodules of clover from western and eastern Azarbaijan were compared with two imported Australian strains (TAl and WU95). Nitrogen fixation capacity was measured for two cultivars of clover, Trifolium alexandrium (Karaj and Winterpass). The effectiveness of TAl was significantly higher than the WU95 and native strains. The imported and native strains of rhizobia significantly differed with respect to the two clover cultivars. The time required for nodulation was influenced by strains of rhizobia. Nodules were first visible 14 days after inoculation by TAl, but they required 17 and 19 days for clover inoculated by native and WU95 strains, respectively. There was no difference between the two native strains. The result obtained may provide a basis for agricultural exploitation of N2 fixation in Iran.