Podbel’skogo, St. Petersburg, 196608 Russia, e-mail olony@yandex.ru Acknowledgements: Supported by Russian Foundations for Fundamental Research (grant ¹ 14-04-01441), EurAsEC Program (Contract 14.М04.12.0011) and Russian Science Foundation (grant ¹ 14-26-00094) A b s t r a c t The most important traits of nodule bacteria (rhizobia), which are tested for selection of commercially valuable strains are the symbiotic efficiency (ability to increase the mass of the inoculated host plants due to intensive fixation of molecular nitrogen) and competitiveness (ability to inoculate the plants in the presence of other rhizobia strains of the same species). <...> By PCR analysis of alfalfa rhizobia ( phism of natural populations for genes Smc03879 or biosis, encodes for synthesis of poly-β-hydroxybutyrate) and Sma0907 or competition for nodultion). <...> The population of the Uralsk region of Kazakhstan living in the impoverished saline soil, gene was detected in 100 % of the strains, while population of the rich Chernozem soil of the Ternopil region of Ukraine in only 82.2 % of the strains. <...> This polymorphism may reflect a low adaptive value for conversion of Ccompounds into poly--hydroxybutyrate for rhizobia strains from the Ternopil region in comparison with the strains of the Uralsk region. <...> Participation of gene ciency is confirmed by the fact that the gene in the control symbiotic effi, which encodes for the structure of the small subunit of nitrogenase, but does not affect the intensity of its synthesis, differences in polymorphism between strains of two populations were not identified. <...> The absence of polymorphism for gene Smb21375 or in all studied populations may be due to the fact that it encodes an ABC transporter required for the uptake by bacteria the broad range of essential nutrients obtained from both the host plant or from the soil. <...> An increased incidence of effective symbionts among strains lacking gene , indicates the prospects to use the molecular probes containing this gene for the selection of practically important rhizobia genotypes that can provide the plant host with the significant amounts of fixed nitrogen. <...> Keywords: legume-rhizobia symbiosis, nodule bacteria, nitrogen fixation, symbiotic efficiency, competitiveness, population polymorphism, PCR analysis. <...> Using transposon (Tn5) mutagenesis in alfalfa rhizobia ( ), a collection of -gene mutants that increase bacterial symbiotic efficacy (SE), i.e. their ability to increase productivity (biomass, the amount of accumulated nitrogen, seed yield) in inoculated <...>