Previously, we have shown that theGlycine max(L.) Merr. -Bradyrhizobium japonicumsymbiosis is very sensitive to inhibition by NH4+. The current study addresses whether the supernodulating soybean mutant,nts382, which is known to be tolerant of NO3-, is also tolerant of NH4+. Thents382 mutant and its wild-type parent, Bragg, were grown in continuous-flow hydroponic culture in the presence of 0, 0.25, 0.5, or 1.0 mM15N-enriched NH4+. Plants were harvested at 14, 21, and 28 days after inoculation. Both cultivars had the highest dry weight (DW) at each harvest date when grown on 0.25 mM NH4+. At 0.25 mM NH4+, whole plant DW increased by 5.3- and 3.2-fold in Bragg andnts382, respectively, compared with the 0.0 mM NH4+control by the end of the experiment. As expected, whole-plant nodulation (nodules per plant), DW-specific nodulation (nodules per gram root dry weight), and nodule DW were severely inhibited in Bragg at all levels of NH4+. However, innts382, whole-plant nodulation was not affected by NH4+treatment, and nodule DW increased by as much as fivefold. Whereas DW-specific nodulation decreased by 94% in Bragg, this parameter decreased by only 52% in thents382 mutant. Likewise, while the nitrogen derived from the atmosphere decreased by approximately 40% in NH4+-supplied Bragg, it increased 2.8-fold at 0.25 and 0.5 mM NH4+innts382. This study demonstrates that both nodulation and N2fixation innts382 are more tolerant of NH4+than in the wild-type Bragg.