The influence of the time of inoculation with an indigenousRhizobiumstrain on biological nitrogen fixation in soyabean was tested in a field experiment in Greece, in an area extremely low in indigenousRhizobiaeffective on soyabean. Inoculation was either at sowing by seed inoculation or 30 days later with liquid solution. One nodulating and one non-nodulating variety ofGlycine max(L.) Merr., were used, while the contribution of the low indigenousRhizobiumpopulation was measured with uninoculated plants of the nodulating variety. Nitrogen fertilizer labelled with15N enabled the nitrogen derived from fixation (% NdfF), from fertilizer (% Ndff) and from the soil (% Ndfs) to be estimated.