Incubation of Xanthomonas campestris pv. citri, pv. campestris, pv. pruni, and pv. vesicatoria with an extracellular O-2 generating system, xanthine-xanthine oxidase (XOD) reaction mixture, resulted in extensive death of these bacteria. However, other tested bacteria, Agrobacterium tumefaciens, Erwinia herbicola and Corynebacterium michiganense were more tolerant to the O-2 generating system. The toxicity of the O-2 generating system to each bacterium was positively correlated with sensitivity to ultraviolet light irradiation, and negatively correlated with the activity of the bacterial superoxide dismutase (SOD). Killing of X. campestris pv. citri by the O-2 generating system occurred with a linear dependency on log concentration of XOD. Exogenously added SOD partially negated the effect with a linear dependency on the log concentration. X. campestris pv. citri was also highly sensitive to methylviologen, which is known to produce superoxide in living cells. Co-injection into citrus leaves of a virulent isolate of X. campestris pv. citri together with methylviologen, at a concentration causing no host tissue damage, resulted in marked inhibition of the bacterial growth in the host leaf tissue. Co-injection with SOD and catalase restored the growth of the bacteria. A spray application of methylviologen onto citrus leaves, which was sprayed at 1, 7, and 24hr after the bacteria were inoculated by spray, protected the leaves from occurrence of citrus canker lesions.