The requirement for molybdenum in biological dinitrogen fixation, first reported by Bortels1, is due to its involvement at or near the site of reduction of N2 in conventional nitrogenase. To date, all nitrogenases which have been purified to homogeneity consist of an iron protein (component 2) and a molybdoprotein (component 1)2. Azotobacter vinelandii, an obligately aerobic diazotrophic bacterium, has two systems for nitrogen fixation: a conventional nitrogenase involving molybdenum and an alternative system which functions under conditions of Mo deficiency and does not require the structural genes for conventional nitrogenase3-6. The properties of the nitrogenase in extracts of comparable deletion strains of A. vinelandii are consistent with a two-component system6,7 in which the component 1-containing fraction has no detectable Mo (ref. 6). Recently, an alternative nitrogen fixation system has been demonstrated in Azotobacter chroococcum strain MCD1155, in which the structural genes for conventional nitrogenase are deleted8. We demonstrate here that nitrogen fixation by this strain depends on vanadium and we show that its purified nitrogenase is a binary system in which the conventional molybdoprotein is replaced by a vanadoprotein.