Hydatid disease (echinococcosis) has a two-host cycle involving the domestic dog and grazing animals. Humans are also infected by the dog. Both unilocular (Echinococcus granulosus in yaks, sheep, and goats) and multilocular (alveolar) (E. multilocularis in hares and rodents) hydatids are common in western Sichuan in the People's Republic of China. Humans and dogs are equally infected with both species. Many yaks (Bos grunniens) were found with multilocular cysts that visually were deemed to be E. multilocularis. However, a histologic and molecular study showed that they were actually E. granulosus. No infective cysts were found in 125 necropsied yaks. We conclude that the yak is an inadequate and dead-end host for the sheep dog (G1) strain of Echinococcus granulosus and also for E. multilocularis.