Quinclorac bensulfuron-methyl is a mixed herbicide widely used on paddy rice field to effectively control barnyard grass and most broad-leaved grasses and sedges. We analyzed superoxide dismutase (SOD) and catalase activities in the quinclorac-highly degrading strain Stenotrophomonas maltophilia WZ2 and Gram-negative standard strain Escherichia coli K12 in an attempt to understand antioxidant enzymes in bacteria are produced in response to quinclorac or bensulfuron-methyl, which increases the virulence of the bacteria. MnSOD and two additional catalase isozymes were induced by quinclorac or bensulfuron-methyl in S. maltophilia WZ2, but not in E. coli K12. Quinclorac turned out to be a more sensitive inducer of SOD, whereas bensulfuron-methyl is a more sensitive one of catalase. A mixture of both has effects similar to quinclorac. Results indicate that catalase has a much weakly role in the defense against quinclorac or bensulfuron-methyl induced oxidative stress, whereas SOD could be critical.