Pot and test-tube experiments were carried out on the effect of phages on yield and nitrogen fixation in lucerne inoculated with effective and non-effective strains of Rhizobium meliloti. It was found that some strains of R. meliloti became activated by the phages, which was reflected in an increase in yield and in crude protein content in the plants, while others lost their effectiveness. The results of pot experiments were in line with those of the test-tube experiments and have demonstrated that the effect of phages on the effectiveness of Rhizobium is not simply negative or positive, but depends on the properties of the bacterial strains used.