The intervarietal variation of the activity of symbiotic nitrogen fixation was studied in soya 25, 39 and 53 days after germination. The highest phenotypical variability of the nitrogen-fixing activity was observed in 39-day plants. The dispersion analysis determined indices that showed the intensity of the effect of nodule bacteria genotype, and strain and their combination on the efficiency of symbiotic nitrogen fixation. The estimation of the acetylene-reducing activity of the symbiotic system in 39-day plants is shown to be the most promising for the breeding purposes.