Growth, nodulation, nitrogen fixation and transfer of N2were studied in a mixed cropping system of cowpea (Vigna unguiculataL. (Walp)) and rice (Oryza sativaL.). The treatments consisted of three cropping systems which were: monocrop cowpea, monocrop rice and cowpea/rice mixed cropping.l5N-labelled ammonium sulphate (20 kg N ha−lof atom %l5N excess) was applied evenly in solution to thel5N subplots while the same amount of unlabelled ammonium sulphate was applied in salt form to thel4N subplots. The cropping system, either sole or mixed with rice, did not significantly influence nodule dry weight at any of the sampling dates. Plants of sole cowpea had a higher shoot dry weight than mixed-crop cowpea plants while the growth of rice was essentially the same. Sequential harvests made at 25, 35, 45 and 55 days after planting (DAP) showed that the proportion of nitrogen derived from N2fixation and the amount of N2fixed by sole and mixed-crop cowpea were similar. Thel5N enrichment data show no clear evidence of significant N2transfer in the cowpea/rice mixed cropping system. The fact that the15N enrichment in the mixed rice was not significantly different from that of sole rice suggests that the15N enrichment in the mixed rice was not diluted by fixed N2from cowpea.