Ancylobacter sp. XJ-412-1, capable of degrading metsulfuron-methyl, was isolated from sulfonylurea-contaminated soil. When metsulfuron-methyl was provided as the sole carbon source, more than 90.5% of metsulfuron-methyl at concentration of 50 mg l(-1) was degraded by strain XJ-412-1 after incubation at 30A degrees C for 7 days. The initial degradation products of metsulfuron-methyl (MSM), thifensulfuron-methyl (TSM), and bensulfuron-methyl (BSM) by XJ-412-1 were identified as corresponding deesterified derivatives by liquid chromatography-mass spectrometry, which indicated a primary pathway of the deesterification of these three sulfonylurea herbicides. The carboxyesterase activity of the cell-free extracts was assayed and strongly inhibited by 4-chloromercuribenzoic acid (PCMB), diethyl pyrocarbonate (DEPC), phenylmethylsulfonyl fluoride (PMSF), and malathion.