A comparison between the supernodulating soybean mutant (nts382) and its parent cultivar (Bragg) under different levels of irradiance revealed a substantially decreased shoot and root (minus nodule) biomass production as a consequence of higher carbon demand of the greater nodule complement carried by nts382. At high irradiance this coincided with an increased specific nodule activity. At low light conditions, nodule activity decreased whereas carbon demand remained high due to the presence of a large number of small nodules with low contents of haem and soluble sugars. Diminished sugar levels in nodules indicated a restricted carbon flow from the shoot. In contrast, haem and sugar content of the wild type remained essentially unchanged in response to the treatments. Measurements of nitrogenase activity under elevated pO2 concurred with the concept of carbon limitation in the mutant under low light conditions in providing no evidence for O2 limitation of N2 fixation activity. The results emphasize the lack of nodulation control by the mutant, but not its parent cultivar, even under severely adverse conditions.