Evolution of hydrogen (H2) gas during N2 reduction in root nodules results in inefficient use of energy needed for N2-fixation. Cultivars of pigeon pea [Cajanus cajan (L.) Millsp.] were inoculated with Rhizobium strains with and without genes for uptake hydrogenase (Hup) activity. Hydrogen evolution acetylene (C2H2) reduction, and Hup activities were assayed on resulting nodules. Plants inoculated with Hup+ Rhizobium strains produced significantly higher yields than those with Hup- strains. Hydrogen-uptake activity by the strain was influenced by the cultivar characteristics. Expression of the plasmid-borne hup genes (pHU52) of Bradyrhizobium japonicum was modified by the host cultivar. The average nodule fresh weight and shoot and root dry weights of the cultivars significantly increased following inoculation with the transconjugant Hup+ Rhizobium strain. Thus, biological N2-fixation may be enhanced by selecting Rhizobium strains that are appropriately matched to the particular cultivar. Incorporation of transconjugant Hup+ genes can increase the rhizobial activity.