Locoweed (O. sericea) poisoning was confirmed in 16 free-ranging elk (C. e. nelsoni) from northern New Mexico [USA] during 1977-1981. Clinical signs consistently seen were emaciation, weakness, incoordination, muscular trembling, posterior ataxia, lethargy and visual impairment. Gross morphologic changes included hydrothorax, hydroperitoneum, hydropericardium, meningeal edema, serous atrophy of fat deposits and anemia. Consistent histological changes were widespread cytoplasmic vacuolation in the parenchyma of most major organ systems. This outbreak of locoweed poisoning coincided with poor range condition exacerbated by subnormal precipitation, and was not considered to be a significant mortality factor in the elk herd. Locoweed poisoning may significantly affect population dynamics of elk herds restricted to ranges severely infested by locoweed.