Xanthomonas leaf spot is a new disease that has occurred on catnip (Nepeta cataria). Catnip is grown commercially in California for use as herbs, seasonings, and tea. This disease has developed recently on catnip transplants that are produced in enclosed greenhouses. Symptoms consist of small brown flecks that are visible from both sides of a leaf. The flecks later develop into larger, dark brown, angular leaf spots. Severe infection reduced the quality and marketability of the transplants. Xanthomonas campestris, as identified by biochemical, physiological, and molecular tests, was consistently isolated from symptomatic plants, and selected strains caused similar symptoms when inoculated onto catnip test plants. However, catnip strains failed to cause any symptoms when inoculated onto nine other plants in the Lamiaceae family and five other hosts of known X. campestris pathovars. Catnip plants showed no symptoms when inoculated with X. campestris pvs. campestris, carotae, and vesicatoria. Catnip also was not susceptible to the X. campestris pathogen isolated from lavender. This is the first report of a bacterial disease of catnip caused by a Xanthomonas pathogen, and the catnip strains may be a new and distinct pathovar of X. campestris.