Based on an analysis of the results of previous studies, a review of theoretical and practical data is devoted to the study of salt tolerance and genetic determination of this trait in nodule bacteria Sinorhizobium meliloti - alfalfa symbionts. Extensive modern data are presented on genes that affect the adaptivity of proteobacteria and on groups of genes which activity varies depending on the osmolarity of the medium. The structural and functional polymorphism of bet genes involved in the synthesis and transport of betaines in S. meliloti is discussed. The phenotypic and genotypic polymorphism of 282 natural rhizobia strains isolated at alfalfa diversity centers under the influence of aridity and salinity is considered.