The organization of genes with the capacity to code for four proteins involved in nitrogen fixation in Frankia strain FaC1 was determined by restriction fragment mapping and nucleotide sequence analysis. Analysis of the 44-kb genomic cosmid clone pFAH 1. isolated from a cosmid library made from Frankia strain FaCl, resulted in the identification of a 7.2-kb PstI fragment to which Klebsiella nifH, nifD and nifK probes hybridized. This nif-hybridizing fragment was subcloned and analyzed by restriction fragment mapping. Further subcloning of the 7.2-kb fragment and subsequent sequence analysis of approximately 6.8 kb revealed the presence of six open reading frames (ORFs). Four of these ORFs have the potential to code for nifV-, nifH-, nifD- and nifK-like gene products and the two others are unidentified ORFs. The organization of the structural genes for nitrogenase is the same in this Frankia strain as it is in most other nitrogen-fixing prokaryotes, but the positioning of the nifV-like gene relative to the nifHDK cluster differs, A consensus nif-promoter-like sequence, found 5’to nifH. was not detected upstream of the nifV-like gene. Nine copies of a 7-bp direct repeat were found 5’to ORFA.