A new nitrogenase from Azotobacter vinelandii has been isolated and characterized. It consists of two proteins, one of which is almost identical with the Fe protein (component 2) of the conventional enzyme. The second protein (Av1'), however, has now been isolated and shown to differ completely from conventional component 1, i.e., the MoFe protein. This new protein consists of two polypeptides with a total molecular weight of around 200,000. In place of Mo and Fe it contains V and Fe with a V:Fe ratio of 1:13 +/- 3. The ESR spectrum of Av1' also differs from conventional component 1 in that lacks the g = 3.6 resonance that arises from the FeMo cofactor but contains an axial signal with gav less than 2 as well as inflections in the g = 4-6 region possibly arising from an S = 3/2 state. This new enzyme can reduce dinitrogen, protons, and acetylene but is only able to utilize 10-15% of its electrons for the reduction of acetylene.