Tryptophan‐catabolic variants, tan 4b and 18ac, of Bradyrhizobium japonicum USDA 26 were isolated from enrichment cultures in nitrogen (N)‐limited media containing either ammonium or glutamate. Presence of exogenous tryptophan (trp) in the medium led to sparse restricted growth and elicited selective growth of tan‐coloured variants over that of parental USDA 26. A 36% increase in cellular uptake‐accumulation of the ammonia analogue [14C]‐methylamine was found with trp‐induced tan 18ac cells over that measured with either ammonium‐induced tan 18ac or trp‐induced tan 4b. The assimilation patterns of uniformly labelled [14C]‐glutamate also differed when tan 18ac was compared with tan 4b. These studies of tan variants isolated from enrichment cultures suggest that bradyrhizobial populations can be manipulated by changing the N sources that limit their growth.