DNA sequence homology with the plasmid pSA30 carrying the cloned nifHDK genes from Klebsiella pneumoniae was revealed in ten Frankia spp. strains, nitrogen-fixing symbionts of non-legumes, irrespective of the strain phenotype (Nod+Fix+, Nod+ Fix- or Nod- Fix?). None of the Frankia spp. DNAs exhibited homology with the plasmid pRmSL26 harbouring the Rhizobium meliloti nod-genes encoding for early symbiotic functions.