We studied the effects of molybdenum, vanadium, and tungsten on the diazotrophic growth of Methanococcus maripaludis. Mo stimulated growth, with a maximal response at 4.0 microM, while V had no effect at any concentration tested. W specifically inhibited diazotrophic growth in the presence of Mo. Coupling the results of our analysis and other known metal requirements with phylogenies derived from nifD and nifK genes revealed distinct clusters for Mo-, V-, and Fe-dinitrogenases and suggested that most methanogens also have molybdenum-type nitrogenases.