Recently, it has been demonstrated that both A. vinelandii and A. chroococcum have the ability to synthesize several different nitrogen-fixing enzymes. Both species can produce a Mo- or V-containing nitrogenase while A. vinelandii can also generate an all-Fe form of the enzyme. Regardless of the source or form of the enzyme, all nitrogenases are composed of two separable proteins, called components 1 and 2, where component 2 is a highly conserved dimer containing a single [4Fe-4S] cluster. The major differences among the various forms of the enzyme are in component 1, the protein where substrate reduction occurs. This protein can exist in forms of four, five, or six subunits and can contain Mo, V or only Fe at the putative active site. Finally, there are also variations among the different enzyme systems regarding the paramagnetism of component 1 as well as the general substrate reduction patterns. While it is not yet known why these bacteria possess the ability to generate multiple forms of nitrogenase or which form of the enzyme has the greatest physiological importance, the existence of these various nitrogenases raises several other important questions. Specifically, how do Mo, V and Fe regulate the synthesis of each enzyme system, which genes are unique and which are common for these systems, and what roles, if any, do Mo, V and Fe play in catalysis? The fact that we can now use three different enzyme systems to investigate nitrogen fixation may greatly increase our ability to answer these questions and eventually understand the mechanism of this very important enzymatic reaction.