The caseinolytic protease (Clp) system is essential for survival under stress conditions and for virulence in several pathogenic bacteria.Xanthomonas campestrispv.campestris(Xcc) is a plant pathogen which causes black rot disease in crucifers. In this study, the XccclpPgene which is annotated to encode the proteolytic core of Clp was characterized. Mutation ofclpPresulted in susceptibility to high temperature and puromycin stresses. Site-directed mutagenesis revealed that S105, H130, and D179 are critical amino acid residues for ClpP function in puromycin tolerance. Inactivation ofclpPalso revealed an attenuation of virulence on the host plant and a reduction in the production of extracellular cellulase, mannanase, pectinase, and protease. The affected phenotypes of theclpPmutant could be complemented to wild-type levels by the intactclpPgene. Transcriptional analysis revealed that expression ofclpPis induced under heat shock condition.