Until recently the molecular genetics of N2 fixation was an almost completely unexplored area. Now, however, it has begun to yield to the classical techniques of bacterial genetics, and the foundation of this important aspect of N2 fixation is taking form. The most noteworthy achievement to date, reported by DIXON and POSTGATE (180), is the transfer of genes for dinitrogen fixation (nif) from Klebsiella pneumoniae to Escherichia coli. Additionally, substantial progress has been made in the mapping of the nif genes (671, 673). Prior to this recent work on Klebsiella, genetic interest in N2 fixation at the molecular level did not extend much beyond the procurement of nitrogenaseless mutants (below) or the development of improved Rhizobia-legume relationships (48). Such studies provided no insight into the nature of nitrogenase genetics per se. In contrast the investigations with Klebsiella, though just under way, have already yielded important results bearing directly on nitrogenase genetics and have established the base for continued investigation.