Greenhouse experiments were conducted to determine the effect of rate and timing of nitrogen (N) fertilizer on growth, N uptake and N2 fixation by nodulated field beans (Phaseolus vulgaris L. ’GN1140’). Fertilizer N was added at 30, 60 or 120 mg kg−1 soil either at planting or at 2, 4, 6, 8 or 10 wk after planting. N2 fixation was determined by using 15N isotope dilution methods with spring wheat (Triticum aestivum ’Leader’) as a nonfixing control plant. Additions of N at 30 mg kg−1 soil had a stimulatory effect on plant growth, relative to plants not receiving N fertilizer, which was reflected in increased N uptake and N2 fixation. Addition of N at 60 or 120 mg kg−1 soil did not result in increased plant N uptake and was shown to inhibit N2 fixation. Stimulatory effects of 30 mg N, and inhibitory effects of 60 or 120 mg N, were only observed if N additions were made within the first 6 wk after planting. Additions of N after that time did not affect the plant parameters measured in this study. It was concluded that additions of N at rates of 60 or 120 mg kg−1 do not result in increased plant growth because of the resulting decreases in the contribution of biologically fixed N2 to plant N uptake. It was also concluded that once the N2-fixing symbioses with GN1140 was established, biological N2 fixation was able to supply sufficient N for the plant needs.Key words: 15N dilution, starter N, field bean, N2 fixation, N addition, wheat, Rhizobium phaseoli