In all organisms that fix atmospheric nitrogen a by-product of the nitrogenase reaction is hydrogen gas1 which may dissipate up to one-third of the energy flux through nitrogenase2. Some nitrogen-fixing bacteria, including certain strains of the root nodule bacterium Rhizobium, possess an active hydrogen uptake (Hup) system permitting hydrogen to be re-cycled3,4. For this reason Hup+ Rhizobium strains are thought to be more energy-efficient symbionts than their Hup− counterparts5–8. We report here that determinants for hydrogenase activity (hup) in a particular strain of R. leguminosarum (128C53) are genetically linked to determinants for nodulation ability (nod), and are probably carried on a plasmid, pRL6JI, of molecular weight (MW)∼19 × 107. Although pRL6JI was not self-transmissible, the determinants for nodulation ability and hydrogenase activity (hup) could be transferred to other strains of R. leguminosarum after recombination with a derivative of a transmissible R. leguminosarum plasmid.