Alfalfa ( Medicago sativa ), as an autotetraploid, allogamous and important forage legume, develops symbiotic association with its bacterial symbioant Sinorhizobium meliloti . Despite considerable capacity for nitrogen fixation, the abundance of rhizobia in alfalfa growing area does not necessarily result in yield increase. Evidences suggest that the success of this symbiosis is influenced by the dynamics of indigenous rhizobial populations, their survival and competitiveness along with several factors, such as location, host cultivar and individual plant genotype. To determine diversity of Sinorhizobium strains nodulating Iranian alfalfa genotypes, inorhizobium populations from eight different Iranian sites were sampled using two Iranian (Hamadani and Nikshahri) and an American (Kodi) alfalfa cultivars as trap host plants.. A total of 982 rhizobial strains were isolated and species were identified showing large prevalence of S. meliloti over S. medicae . Analysis of salt tolerance and melanin production revealed high diversity among strains, whereas significant differences were not observed between groups of strains isolated from different sites and cultivars. Of S. meliloti strains collected in this study nine were indexed as rhizopine isolates using specific primers designed from rhizopine catabolism gene mocA and GC-MS analysis. It is believed that rhizopine producing rhizobia confer competitive ability in their interactions with the diverse microbial community of rhizosphere. Genetic diversity of the Sinorhizobium isolates was analysed by BOX-PCR and ERIC. Analysis of molecular variance (AMOVA) based on BOX-PCR data was performed to partition total molecular variance into variance contributed by plant variety and soil types. The results indicated that most of the total molecular variance was attributable to divergence among strains isolated from the same sites and cultivars (intra-population genetic variance). A significant genetic difference (1.97%) were found among sites; in contrast, only 0.77% of the genetic diversity was attributable to differences among cultivars showing that the effect of different sites of origin could be more relevant in shaping population genetic diversity than the effect of different cultivars and individual plants. To examin the competitiveness and ability of chosen strains of S. meliloti to nodulate three local populations (Hamedani, Yazdi , Nikshahri) and an American cultivar (Kodi) of alfalfa, the selected strains with exclusive fingerprinting patterns including strain 5A1 (salt tolerant strain with rhizopine catabolism activity), 6A23 Key Words: Medicago sativa , Sinorhizobium meliloti , S. medicae , Diversity, Rhizopine, Competitiveness.