A Rhizobiummeliloti DNA region, determining nodulation functions common in different Rhizobium species, has been delimited by directed Tn5 mutagenesis and its nucleotide sequence has been determined. The sequence data indicates three large open reading frames with the same polarity coding for three proteins of 196, 217 and 402 (or 426) amino acid residues, respectively. We suggest the existence of three nod genes on this region, which were designated as nodAB and C, respectively. Comparison of the R.melliotinodA, B, C nucleotide and amino acid sequences with those from R.leguminosarum, as reported in the accompanying paper, shows 69–72% homology, clearly demonstrating the high degree of conservation of common nod genes in these Rhizobium species.