The nifH gene encodes the iron protein (component II) of the nitrogenase complex. We have previously shown the presence in Clostridium pasteurlanum of two nifH-like sequences in addition to the nifHl gene which codes for a protein identical to the isolated iron protein. In the present study, we report that there are at least five nifH-like sequences in C. pasteurlanum. DNA sequencing data indicate that the six nifH (nifHl) and nifH-like (nifH2, nifH3, nifH4, nifH5 and nifH6) sequences are not identical and vary from each other to different extents with sequence identity ranging between 68 to 99.9% within the nifH coding regions. Under normal N2-fixing growth conditions (molybdenum-containing medium), transcripts of nifH1 and most of the nlfH-llke sequences accumulate. The above results suggest the functioning of more than one "nifH" gene under N2-fixing growth conditions for C. pasteurianum. A common sequence was found around the -100 regions of all nif or nif-like transcription units. Sequences identical to or very similar to the consensus Escherichia coli promoter were found in the -35 and -10 regions. © 1988 IRL Press Limited, Oxford, England.