Adult Haemaphysalis qinghaiensis ticks from Gannan area, where ovine theileriosis is prevalent but bovine theileriosis has not been reported, transmitted Theileria parasites to yaks and sheep. Blood from the infected yak contained Theileria parasites that were infective to yaks, but not sheep. In contrast, blood from the infected sheep did not contain Theileria parasites infective to yaks. These results suggested that the ticks from the Gannan area were infected with two species of Theileria one which specifically parasitises yaks, one which parasitises sheep. Both infections were more aggressive in splenectomised hosts as compared to intact host. Adult H. qinghaiensis ticks from Zhanjiachuan area, where ovine theileriosis is prevalent but where no yaks are raised, transmitted Theileria parasites to ovine, but not yaks. Taken together these results indicate that H. qinghaiensis ticks may be infected with two different Theileria species. The species infective for sheep could be related to the newly recognised, but not yet named, pathogen recently reported in small ruminants in China. The species infective for yaks appears to be benign and is morphologically similar to parasites of the Theileria sergenti/orientalis/buffeli group.