An epidemiological investigation on echinococcosis was made in Jiuzhi County of Qinghai Province, western China. Ultrasonography and an indirect hemagglutination test revealed a morbidity of 8.0% (124/1,549) and a seroprevalence of 25.8% (287/1,113), respectively, in the Tibetan population. The morbidity in herdsmen (16.6%) and Buddhist priests (15%) was significantly higher than that in other occupation groups (3.2%), and it was higher in females (9.8%) than in males (6.2%). The ultrasound images showed a coexistence of cystic echinococcosis (CE) and alveolar echinococcosis (AE), occupying 69 and 31% of the cases, respectively. An Echinococcus Western blot assay was performed as a serological backup test for differentiating CE and AE. The assay revealed that serum samples from most cases with a positive AE image showed a specific antibody against antigen bands at 16/18 kDa. Autopsy proved that 9 out of 12 stray dogs were infected with Echinococcus granulosus (n = 8) and E. multilocularis (n = 1). Inspection at the abattoirs demonstrated a hydatid rate of 78.5% in yaks and 82.6% in sheep. The data indicate that Jiuzhi County is an important endemic area for both CE and AE, in both human and animal populations.