Allozyme electrophoresis was carried out in starch gel to compare the electrophoretic patterns for acid phosphatase (AcP, EC 3. 1. 3.2), peroxidase (Po, EC 1.11.1.7) and esterase (Est, EC 3. 1. 1. 1). It was found in this study that isoenzymes AcP1, AcP2 and Po1, Po2 were coded by 2 loci respectively and they were monomorphic. It is possible to determine 4 loci responsible for esterase zymogram, among which isoenzyme Est3 region either consisted of a fast (Est3-F) band in the case of Oncomelania hupensis isolated from Anhui, or a slow (Est3-S) band in snails from Yunnan. The Rf values of the fast and slow bands were 0.362 +/- 0.027 and 0.340 +/- 0.036, respectively. Snails from Yunnan, at the upper reaches of the Yangtze River, are proved to be resistant to infection with Anhui-Hubei strain of Schistosoma japonicum in the mainland of China, while snails from Anhui, at the lower reaches of the Yangtze, susceptible to infection with Anhui-Hubei strain of S. japonicum.