Free-living bacteria fixing-N₂ (from genera of Azospirillum and Acinetobacler-like) occurred in the rhizosphere of barley, cultivated on different soils and also in the rhizosphere of maize and grasses grown on medium alluvial soil. In the rhizosphere of barley occurrence of Azospirillum sp. strains was found in 84%, and in the rhizosphere of maize and grasses in the amount of 100% and 94%, respectively. Bacteria of the genus Azospirillum were not found in the rhizosphere of Lolium perenne, Phleum prateme and Poa palustris. In the rhizosphere of barley cultivated on rendzinas and black earth developed from slightly loamy sand and in the rhizosphere of grasses, were found of halophylic strains of Azospirillum sp. (growth in medium with 3% NaCl) and strains, which preliminary determined as Acinetobacler-like (growth in medium with 6% NaCl). The most probable number (MPN) of bacteria from the genus Azospirillum counted in the rhizosphere of barley, maize and grasses ranged from 0 to 2450 cells g^–1 of soil dry matter. No correlation between the frequency of their occurrence and content of available components, humus and exehangeable cations in the soil sampling date was observed. The nitrogenase activity of Azospirillum spp. strains isolated from the barley, maize and grasses rhizosphere ranged from 1.6 to 228.5 nM C₂H₄ h^–1 cm^–3 of gas phase. The nitrogenase activity of the isolated Acinetohacter-like strains ranged from 8.4 to 59.4 nM C₂H₄ h^–1cm^–3 of the gas phase.