The influence of vesicular - arbuscular Mycorrhiza fungi (VAM) on symbiotic fixation of N٢ in alfalfa plants has been observed. Beneficial effects of VAM plants in saline soil are of great interest. The aim of this work was to study the effect of VAM or phosphorous fertilization on alfalfa (Medicago sativa L.) yields, number of nodules and N٢ fixation by N١٥ isotope dilution at different salinity levels.