Control of downy brome in cool-season perennial grasses grown for seed production must be done prior to seed harvest, because downy brome seeds cannot be separated from cool-season grass seeds. Field experiments were conducted near Powell, WY, and Bridger, MT, to evaluate several herbicides for control of downy brome from 1992 through 1994 in western wheatgrass, slender wheatgrass, beardless wild rye, thickspike wheatgrass, and meadow bromegrass. When fall-applied, only metribuzin at 0.4 kg/ha and oxyfluorfen plus metribuzin at 1.1 plus 0.3 kg/ha controlled 98 and 95% of the downy brome, respectively, without affecting grass seed viability or seed yield. Early spring applications of paraquat at 0.8 kg/ha controlled 100% of the downy brome, but suppressed perennial grasses. Glyphosate applied in early spring at 0.3 kg/ha controlled 48% of the downy brome, but suppressed perennial grasses. Seed yield reductions occurred when thickspike wheatgrass and meadow brome were treated with glyphosate at 0.3 kg/ha.