This paper reports effects of molybdenum and vanadium on Anabaena variabilis Kutz. (ATCC 29413) and Anabaena cylindrica Lemm. (PCC 7122), previously grown in NH4+-containing medium deficient in Mo and V (−Mo −V) and then transferred to media lacking combined nitrogen. Diazotrophic growth of A. variabilis was dependent on the presence of Mo or V, with little growth occurring in their absence. Neither Mo nor V was required for heterocyst differentiation in either cyanobacterium, and the highest heterocyst frequencies occurred in cultures showing the lowest growth rates. Although A. cylindrica also showed Mo-dependent diazotrophic growth and a low rate of growth in the absence of added Mo, the latter was inhibited by addition of 1 μM V. The rate of acetylene reduction by A. variabilis increased rapidly following addition of V to −Mo−V cultures, and increased ethane formation was detected in the acetylene reduction assays. Addition of Mo resulted in a smaller initial increase in the rate of acetylene reduction, but this was also accompanied by increased ethane formation, followed by a slow and much more extensive increase in acetylene reduction with a decrease in ethane formation. These observations indicate that nitrogenase polypeptides, probably those of a V-nitrogenase, are synthesized in A. variabilis in the absence of added Mo or V, that added Mo can be incorporated into the V-nitrogenase, and that synthesis of Mo-nitrogenase is dependent on the presence of Mo. By contrast, A. cylindrica appears either to synthesize only a Mo-nitrogenase, even in the absence of Mo (though 1 μM V inhibits its synthesis) or to synthesize a Mo- and V-independent nitrogenase, but only in the absence of added Mo and V. Western blots using antisera against the Mo-Fe and Fe proteins of the Mo-nitrogenase of Rhodospirillum rubrum indicated the presence of Mo-nitrogenase polypeptides in both Mo-grown and −Mo−V grown A. cylindrica, providing evidence in support of the former possibility. Physiologically, the V-nitrogenase of A. variabilis showed several distinct similarities to the Mo-nitrogenase of A. cylindrica, including responses to metal concentrations as well as the above responses to addition of V and Mo, respectively.