Rhizobium phaseoli CFN42 possesses three regions containing genes coding for the nitrogenase polypeptides: region a and region b contain copies of the three nitrogenase structural genes nifH, nifD and nifK and region c contains only a nifH gene. Gene fusions in which the Escherichia coli lacZ gene is under the control of each of the three nifH promoters were constructed. These fusions have been used to study expression of these genes. All mutant strains carrying any of the three nifH::lacZ fusions as well as double mutants in which only regions a or b remain functional showed β-galactosidase and acetylene reduction activites, indicating that the three nifH genes were transcribed and that transcription occurred simultaneously. Compared with the wild type, mutant strains carrying nifHa ::lacZ or nifHb::lacZ showed a reduced ability to support plant growth in the absence of added nitrogen, suggesting a gene dosage effect of the reiterated nifH genes. We also report a genetic rearrangement that occurs in strains with the nifHc::lacZ fusion that generates a new nifHDK operon.