The effect of the plant growth stimulant bactozole on the growth of Rhizobium leguminosarum bv. viciae 250a and its nitrogen-tolerant mutant M-71 and the synthesis of extracellular carbohydrates was studied. At a low content of nitrate (6 mM) in the medium, all three bactozole concentrations tested (0.001, 0.01, and 0.1%) exerted similar stimulating effects on the growth of the parent strain 250a (about 1.5-fold) and the synthesis of extracellular carbohydrates (about 2-fold). At a high content of nitrate (20 mM) in the medium, when the growth of the parent strain and the synthesis of extracellular carbohydrates were inhibited, bactozole at all three concentrations exerted only a growth-stimulating effect. At the same time, mutant M-71 showed better growth at higher concentrations of bactozole, whereas the ability of the mutant to synthesize extracellular carbohydrates decreased with increasing bactozole concentration. The cell biomass of the mutant accumulated at 20 mM nitrate was 1.8-2.5 times greater than it was at 6 mM nitrate. Bactozole enhanced the symbiosis of legume plants with both parent and mutant strains, raising the mass of plants and enhancing nodulation and the nitrogen-fixing activity of root nodules. The symbiotic parameters of mutant M-71 were better (irrespective of whether bactozole was present or not) when its inoculum was grown at a high nitrogen content (20 mM nitrate), whereas the respective parameters of the parent strain were better when it was grown at 6 mM nitrate. The inference is made that the better physiological characteristics of the mutant in the high-nitrate medium is due to its higher nitrate reductase activity (as compared with the parent strain) in both the free-living state and in legume nodules.