Differences among plant parts in the natural abundance of (15)N are of interest from the point of view of developing a sampling strategy for using (15)N measurements to estimate the contribution of symbiotically fixed N to N(2) fixing plants, and because they reflect isotopic fractionation associated with degradation, transport, and resynthesis of N-bearing molecules. This paper reports such differences in nodulating and nonnodulating isolines of soybeans (Glycine max [L] (Merrill, variety Harosoy)) grown under several different conditions.Nodules were strikingly enriched in (15)N compared to other plant parts (by an average of 8.3 per thousand excess (15)N), and the enrichment increased with time during the growing season. (15)N was much more uniformly distributed among other plant parts. Although there were significant differences among other plant parts, the maximum deviation of the (15)N abundance of any plant part from that of the entire plant was about 2 per thousand (15)N excess. The (15)N abundance of the seed N was most representative of the whole plant.There were significant differences between isolines in the distribution of (15)N. The distribution of (15)N within plants also varied with experimental conditions. The implications of these results for estimation of N(2) fixation from measurements of the natural abundance of (15)N are discussed.