Technical Abstract: 'Mustang' Altai wildrye is recommended for use on arid and semiarid rangelands as a winter forage and a revegetation grass in the Intermountain Region and Northern Great Plains of western U.S.A. The parental germplasm for Mustang was derived from selections within PI 499650 (DT-3185; 79%), 'Prairieland' (7%), 'Eejay' (7%), and 'Pearl' (7%). This cultivar is the result of three cycles of recurrent selection for vegetative vigor, total seed yield, 100-seed weight, and seedling emergence from a 7.6 cm planting depth. Mustang is significantly taller, with longer flag leaves that are oriented higher on the culm than Prairieland, Eejay, and Pearl. In addition, Mustang is green in color with wider flag leaves and longer inflorescences. Mustang Altai wildrye is a dodecaploid (2n -12x - 84) and has the same ploidy level as the commercially available cultivars Prairieland, Eejay, and Pearl. Based on alplified fragment length polymorphisms, the E.ACAG//M.CTTG primer combination distinguished Mustang from the other cultivars. When combined over six locations and three years in the Northern Plains Regional Trials (NPA), Mustang Altai wildrye (3026 kg ha-1) produced significantly more forage dry matter than cultivars Prairieland (2394 kg ha-1) and Pearl (2247 kg ha-1) Altai wildrye, Magnar (2220 kg ha-1) and Trailhead (2214 kg ha-1) basin wildrye [Leymus cinereus (Scribn. & Merr.) A. Love], and Bozoisky-Select (2525 kg ha-1), Mankota (2434 kg ha-1), and Tetra-1 (2118 kg ha-1) Russian wildrye [Psathryostachys juncea (Fisch.) Nevski]. Combined over six locations and three years in the NPA trials, Mustang Altai wildrye had superior seedling establishment (83%) cmpared to Pearl (68%) and Prairieland (64%) Altai wildrye. After four years at Mead and Sidney, NE, Mandan, ND, and Miles City, MT, Mustang (71%) Altai wildrye was more persistent than Prairieland (39%) and Pearl (50%).