Technical Abstract: Slender wheatgrass (Elymus trachycaulus subsp. trachycaulus), is a native North American grass that is used as a livestock forage. Ustilago phrygica, a systemic ovary-smut fungus, is native to Turkey and West Asia, and is pathogenic on Aegilops spp. and Taeniatherum caput-medusae subsp. asperum (medusahead), an invasive weed in the western U.S. that is targeted for biological control. An isolate of the fungus was collected from medusahead in Turkey and screened for possible use in classical biological control of this weed. The focus of screening was determination of host range of the fungus among related native and agriculturally important grasses in North America. A procedure was developed to consistently produce disease on medusahead and other grasses. Teliospores of the fungus were vacuum-inoculated onto seeds of medusahead and slender wheatgrass, which were then placed on moist germination paper in a petri dish or on moist vermiculite in plastic boxes and placed in a dark refrigerator at 3 degrees C. After 8 weeks, all of the seedlings were transplanted into pots on a greenhouse bench. The plants began to flower and produce smutted spikes 40 days later. In total, 23 of 84 medusahead plants from inoculated seeds became smutted and produced numerous smutted spikes per plant. One slender wheatgrass plant out of nine plants grown from inoculated seeds was also smutted and produced three diseased spikes. Our tests with the fungus confirm that slender wheatgrass is susceptible to U. phrygica and lead us to conclude that the fungus would not be a good candidate for classical biological control of medusahead in North America.