Biodiversity survey and inventory have resulted in new information on the distribution of Protostrongylidae in Dall's sheep (Ovis dalli dalli) from the Northwest Territories (NT, Canada) and from Alaska (AK, USA). In 1998, Parelaphostrongylus odocoilei adults were found for the first time in the skeletal muscles of Dall's sheep in the Mackenzie Mountains (NT). Adult P. odocoilei were associated with petechial and ecchymotic hemorrhages and localized myositis; eggs and larvae in the lungs were associated with diffuse granulomatous pneumonia. Experimental infections of the slugs Deroceras laeve and Deroceras reticulatum with dorsal-spined first-stage larvae assumed to be P. odocoilei, from ground-collected feces from Dall's sheep in the Mackenzie Mountains, yielded third-stage larvae by at least 28 (in D. laeve) and 48 (in D. reticulatum) days post-infection. Third-stage larvae emerged from D. laeve between days 19 and 46 post-infection and emergence occurred both at room temperature and at 10 to 12 C. Protostrongylus stilesi were definitively identified from the lungs of Dall's sheep collected in the Mackenzie Mountains, NT in 1998. Specimens collected from sheep in the Mackenzie Mountains, NT in 1971–72, and the Alaska Range, AK in 1972 were also confirmed as P. stilesi. Lung pathology associated with adults, eggs, and larvae of P. stilesi was similar to that described in bighorn sheep (Ovis canadensis). Concurrent infections with P. odocoilei and P. stilesi in a single host have not been previously reported.