The efficacy of copper bactericides for control of Xanthomonas campestris pv. vesicatoria in eastern Oklahoma tomato fields was evaluated. Copper bactericides did not provide adequate control, and copper-resistant (Cu(r)) strains of the pathogen were isolated. The Cu(r) genes in these strains were located on a large indigenous plasmid designated pXV10A. The host range of pXV10A was investigated; this plasmid was efficiently transferred into 8 of 11 X. compestris pathovars. However, the transfer of pXV10A to other phytopathogenic genera was not detected. DNA hybridization experiments were performed to characterize the Cu(r) genes on pXV10A. A probe containing subcloned Cu(r) genes from X. campestris pv. vesicatoria E3C5 hybridized to pXV10A; however, a subclone containing Cu(r) genes from P. syringae pv. tomato PT23 failed to hybridize to pXV10A. Further DNA hybridization experiments were performed to compare pXV10A with pXvCu plasmids, a heterogenous group of Cu(r) plasmids present in strains of X. campestris pv. vesicatoria from Florida. These studies indicated that the Cu(r) genes on pXV10A and pXvCu plasmids share nucleotide sequence homology and may have a common origin. Further experiments showed that these plasmids are distinctly different because pXV10A did not contain sequences homologous to IS476, an insertion sequence present on pXvCu plasmids.