Muskoxen (Ovibos moschlatus moschatus) on the northwestern mainland of Nunavut and Northwest Territories, Canada, are infected with the protostrongylid lungworm, Umingmaksrongylus pallikuuhkensis. The geographic range of this muskox population is expanding to the south and west, and it is anticipated that these animals will eventually become sympatric with Dall's sheep (Ovis dalli dalli) in the Mackenzie and Richardson Mountains. To address the concern of wildlife managers that U. pallikulkensis may infect and adversely affect Dalls sheep, four Dalls/Stone's (Ovis dalli stonei) hybrid lambs and one adult muskox (Ovibos moschatus wardi) were each given 100 third-stage larvae of U. pallikuukensis. All animals were intensively monitored for 9 mo postinfection (PI) using clinical examinations, fecal analyses, hematology, blood chiemistry and medical imaging. No first-stage larvae of U. pallikuinkensis were recovered from the lambs and monitoring revealed nio evidence that the parasite had established in any of these animals. First-stage larvae were found in the feces of the muskox beginning at 94 days PI, and typical parasite cysts were visible in lung radiographs at 188 days PI. This study addresses an important management and wildlife health issue associated with the potential for host-switching of pathogens and indicates that it is improbable that thinhorn sheep are suitable hosts for U. pallikuukensis.