Abstract: 　Objective] To answer the following questions:① For Oncomelania snails collected two years apart from the same locality, has there been genetic divergence? ②How much experimental error has there been in studying subsets of these populations? ③As this is an unstable population, what has the net effect been on Hardy Weinberg equilibrium (HWe)? [Methods]Allozymes were studied using horizontal starch gel electrophoresis. Data collected from numbers of experiments were compiled. Data from each collection were divided into two equal subsets based on chronology of the experiments. Thirty four loci were studied using 72 to 180 snails per subset. [Results] The mean number of alleles per locus ranged from 1 5 ～1 9. With each consecutive subset,the % polymorphic loci dropped from 38 2 to 17 6.The mean heterozygosity was very low: 0 033 to 0 049 and not significantly different from Hardy Weinberg expectations. Ten loci and 11 alleles exclusive to the first group were eliminated from the overall study reducing the number of polymorphic loci from 19 to 10. There were significant departures from HWe at five loci having a substantial number of individuals for each allele. Neis and Wrights D were 0 003±0 001 and 0 054±0 006 respectively. [Conclusion] ① There were significant errors seen primarily in the results scored in the earliest experiments.② These earlier errors involving scoring difficult to resolve loci, and interpretation of rare alleles that were not found in later experiment had no significant effect on overall genetic distance.③ The use of Wright's D for closely related populations is explained. Results with Nei's D indicated no significant difference among the four subunits; Wright's D yielded significant difference between the collections made two years apart, attributed to the annual flooding of the Yangtze River mixing snails from different localities. ④ Major polymorphic loci were not in Hwe as predicted using the unstable population model. ⑤ One must study 25 or more individuals to find relatively rare alleles and study population genetics.

Key words: Oncomelania hupensis, allozyme, unstable population, genetic variation.