In the nitrogen fixing soybean symbiont, Rhizobium japonicum, the genes nifD and nifK for the α and β subunits of dinitrogenase, respectively, and nifH for dinitrogenase reductase are located on separate operons (nifDK and nifH). We report here the transcription start point and the nucleotide sequence of the nifDK promoter and compare it with the nifH promoter. Furthermore, the nucleotide sequences of the complete nifD gene and the beginning of the adjacent nifK gene are presented. The promoter is located in a region from 10 to 31 nucleotides upstream from the transcription start point. It contains a sequence 5′-CTGG-8bp-TTGCA-3′ which has also been recognized as the consensus nif promoter sequence of other nitrogen fixing bacteria such as Klebsiella pneumoniae and Rhizobium meliloti (for refs. see text). On the 5′ side the consensus sequence is immediately preceded by the sequence 5′-GTGC-5bp-AGACC-3′ which appears to be unique to R. japonicum. The nifD gene is 1545 nucleotides long, coding for 515 amino acids which make up a dinitrogenase α subunit with a molecular weight of 57,918. The comparison with the nifD gene product from the cyanobacterium Anabaena (ref. in the text) makes it now possible to identify conserved (and, hence, functionally important) protein domains. Five conserved cysteine residues may be involved in the binding of FeS clusters and/or the MoFe cofactor. On the basis of sequence homologies we discuss the possible evolution of nif genes, and suggest that they may have evolved in parallel with the bacteria that harbor them.