The effect of biological nitrogen fixation, as well as the addition of phosphorus, on three stages of soybean growth, were tested in field experiments in an area of Greece extremely poor inRhizobiaeffective on soybeans. The soil-agar technique permitted the isolation of an effective indigenousRhizobiumstrain from this area, which in turn was used as inoculum in the field experiments. One nodulating variety,Glycine max(L.) Merr, and one non-nodulating as control were used, while nitrogen fertilizer labelled with15N enabled the nitrogen derived from fixation (% NdfF), from fertilizer (% Ndff) and from the soil (% Ndfs) to be estimated.