Successful nodulation of legumes by rhizobia is a complex process that in open field depends on various environmental and biological factors. Generally legume productivity may be improved by inoculation with selected, highly effective in diazotrophy root nodule bacteria. However, field legume inoculation with Rhizobiaceae species is very often unsuccessful due to the presence of native strains in soil which are better adapted and usually dominate over introduced bacteria. The ability of one strain to outnumber others in nodule occupancy is commonly termed competitiveness. This feature of strain is genetically regulated by numerous bacterial genes, as well as it is highly dependent on host plant genotype and environmental cues. The competitiveness of endogenous strains is critical for the successful use of inocula to introduce the quality strains. In this paper we describe ways and means which should be considered in order to manipulate both established and introduced strains ecologically, edaphically and genetically to improve legume productivity and, as the consequence, soil fertility.