Yam bean (Pachyrhizus erosus) a tuber-forming legume, is particularly suitable for evaluating the potential availability of the plant reserve carbohydrates for nitrogen fixation (Vaillant et al., 1990,1993). The tuber of this species contains soluble sugars and starch which account, respectively, for 32 and 15% of dry weight. Furthermore, tuberous and non-tuberous plants may be avai lable in inductive or noninductive environment (Robin et al., 1990; Sorcnsen et al., 1993). A stem cutting experiment was conducted with tuberous Pachyrhizus erosus in order to evaluate its potential for maintaining N fixation after aphotosynthate stress. This paper report the effects of such a treatment on the Acetylene Reduction Assay (ARA) of nodules and on the concentration of N fixation products in underground organs.