Piroplasmosis, caused by tick-borne protozoan parasites of the genera Theileria and Babesia, is one of the most economically important infections of domestic ruminants in China. We evaluated the prevalence of antibodies to Babesia bigemina in wild yaks (Bos mutus) from Qinghai Province, China. Of 91 samples, 24% had detectable B. bigemina antibodies, and significant differences in prevalence were observed between those older than 5 yr and those younger than 5 yr. We collected 1,679 ticks belonging to two Ixodid genera from the infested wild yaks. The most prevalent tick species was Dermacentor nuttalli (48.1%), followed by Haemaphysalis qinghaiensis (33.3%) and Dermacentor silvarum (18.6%). These results indicated that B. bigemina is most likely endemic in the study area, and active surveillance programs for piroplasmosis are needed for monitoring animal health status.