Three captive Rocky Mountain bighorn sheep (Ovis canadensis canadensis), consisting of a female lamb, a yearling ram, and a 2½-yr-old castrated ram were inoculated orally with 50 (n = 1) or 100 (n = 2) metacercariae of Fascioloides magna in November 1991. All three sheep died from fluke infection on post-inoculation days 104, 140, and 197, respectively. Numbers of F. magna recovered were 3 (3%), 18 (36%), and 21 (21%). All flukes were immature and were recovered from liver (n = 36), lungs (n = 2), or peritoneal spaces (n = 4). Two white-tailed deer (Odocoileus virginianus), each were inoculated orally with 100 metacercariae at the same time as the bighorn sheep. Eggs of F. magna were detected in the feces of the deer on postinoculation days 199 and 211, respectively. Both deer remained healthy for the year-long experiment. Thus, bighorn sheep are susceptible to infection with F. magna and are likely to die within approximately 6 months of exposure.