Rhizobium sp. BR816 contains four nodD alleles of which nodD3 is the most important transcriptional regulator for nodulation of Phaseolus vulgaris. Upstream of nodD3 an open reading frame, orf816, was identified. The deduced ORF816 protein shows homology with transcriptional regulators of the AraC/XylS family. The DNA binding domain and the consensus motif, characteristic of the C-terminal region of the members of this family of transcriptional regulators, are present in the deduced ORF816 protein. Activation of nodA gene expression and nodulation of P. vulgaris by Rhizobium sp. NGR234nodD1::omega (Nod-) complemented with the Rhizobium sp. BR816 nodD3 gene were significantly increased in the presence of orf816. This increased nodulation and nod gene induction are mediated through positive regulation of nodD3 expression by ORF816. Expression of orf816 itself is partially RpoN dependent. The role of this transcriptional regulator in the complex cascade regulation of the Rhizobium sp. nodD3 gene is discussed.