Rice (Oryza sativa L.) cultivar Cas 209 carries the gene Xa-10 for resistance to race 2 of Xanthomonas oryzae pv oryzae, the bacterial blight pathogen. When seedling leaves of Cas 209 plants were infiltrated with bacterial cell suspensions of strain PX086RIf (race 2, incompatible), total peroxidase activity in extracts from extracellular spaces increased almost threefold between 16 and 24 hours after inoculation. The increase in total peroxidase activity in extracellular extracts was correlated with the appearance of a 43-kilodalton peroxidase isoenzyme with an isoelectric point of 8.6. Increases in the activities of two anionic peroxidase isoenzymes also were associated with the incompatible interaction. Later during the interactions, total peroxidase activities increased in both compatible (cv Cas 209 infiltrated with race 1, PXO61sm) and control (Cas 209 infiltrated with water) treatments, but final activity levels were less than that observed in the incompatible combination. Similarly, the cationic peroxidase was detected in all three treatments by 48 hours after infiltration, but at reduced levels in compatible and water-infiltrated control treatments relative to the incompatible combination. Accumulation of this peroxidase in extracellular spaces thus may play a role in the defense response in cultivar Cas 209. Rice (Oryza sativa L.) is one of the most important cereal crops worldwide, sustaining more than one-half of the global population (20). Like many plant species, rice employs a diverse array of defenses that minimize losses during pathogen attack. For example, melanin deposition and coordinated increases in peroxidase activities and phenolic compounds are associated with the colonization of rice leaves by Magnaporthe grisea, the blast pathogen (26, 30). Phenolic compounds toxic to Xanthomonas oryzae pv oryzae, the causal agent of bacterial blight, also are found in greater amounts in healthy leaves of resistant cultivars compared to healthy leaves of susceptible cultivars (9). Phytoalexins are thought