The virulence loci play an essential role in tumor formation by Agrobacterium tumefaciens. This study focused on the virC locus, which affects the host range Agrobacterium species. virC mutants display an attenuated or avirulent phenotype on certain host plants, but remain fully virulent on other plant hosts. The nucleotide sequence revealed that the virC locus of pTiA6NC is an operon consisting of two open reading frames. These two open reading frames, designated virC1 and virC2, encode protein products of 25,713 and 22,710 daltons, respectively, which were visualized by polyacrylamide gel electrophoresis. Only two nucleotides separated the stop codon for virC1 from the start codon for virC2, indicating that these genes may be translationally coupled.