Biological dinitrogen fixation by Rhizobium spp. in the root nodules of leguminous plants such as soybean is of considerable agronomic importance. Biological dinitrogen fixation is ATP- and reductant-dependent; between 12 and 30 mol of ATP are required per mol of dinitrogen reduced [1]. All free-living rhizobia are aerobic although some strains will also grow anaerobically with nitrate as the terminal electron acceptor [2] ; ATP and reductant are generated during the oxidation of an exogenously supplied carbon source. In the bacteroids within the root nodule the exogenous carbon source (photosynthate) is derived from photosynthetic CO₂ fixation by the host plant. The identity of the carbon source(s) oxidised by the bacteroids in vivo has not yet been confirmed although sucrose is the major photosynthetic product translocated to the root nodules [3]. As dinitrogen fixation represents a drain on the photosynthetic supply [4,5] and since the supply of photosynthate is probably one of the major factors that limits dinitrogen fixation in the Rhizobium-legume symbiosis [6] a high efficiency of energy coupling within the bacteroids would minimise the amount of photosynthate required to be oxidised to supply the ATP and reductant necessary for dinitrogen fLxation. The number of sites of oxidative phosphorylation in the bacteroids is unknown although Laane et al. [7,8] have found respiration induced membrane energisation in R. leguminosarum bacteroids. We investigated both the capability of the bacteroids to oxidise a variety of carbon sources and the number of sites of oxidative phosphorylation (proton-translocating loops) in the bacteroids.