Abstract Flupyrsulfuron-methyl, methyl 2-[[[[(4,6-dimethoxy-2-pyrimidinyl) amino]carbonyl]amino]sulfonyl]-6-(trifluoromethyl)-3-pyridinecarboxylate monosodium salt, formerly DPX-KE459, is a new postemergence sulfonylurea herbicide for the control of grass and broadleaf weeds in wheat. Similar to other sulfonylureas, the site of action of flupyrsulfuron-methyl is acetolactate synthase (ALS), an enzyme in branched-chain amino acid biosynthesis. Studies of in vitro inhibition of ALS across sensitive and tolerant species discount differential active site sensitivity as the basis for wheat tolerance to this herbicide. However, wheat and the tolerant weeds Avena fatua and Phalaris minor metabolized flupyrsulfuron-methyl very rapidly with half-lives of ≤2 h. Metabolism in the moderately tolerant species Setaria viridis was intermediate (half-life of 10 h), and slow in Alopecurus myosuroides (half-life of 20 h), a sensitive species. The initial metabolic pathway of flupyrsulfuron-methyl in wheat primarily involves glutathione conjugation; whereas in an equally tolerant species, P. minor , this herbicide is not metabolized by glutathione conjugation but apparently by O -demethylation. Flupyrsulfuron-methyl is metabolized via both pathways (glutathione conjugation and O demethylation) in A. fatua , also a tolerant species. Differential rates of metabolism in sensitive and tolerant plant species are responsible for the selectivity of flupyrsulfuron-methyl.