Using starch and polyacrylamide gel electrophoresis, we analyzed the Oariants of II isozymes encoded by 31 presumptive loci in a natural silver carp population from the middle reaches of the Yangtze River. Among the 27 loci usable for genetic population analysis, four (Ldh-C, Idhp-A, Est-1, -2) were polymorphic. The mean proportion of polymorphic loci and the average heterozygosity per locus in the mentioned population were 14.8% and 0.0451, respectively. Comparative result showed that silver carp hatchery and natural populations had a similar mean proportion of polymorphic loci. It was proposed that the reduction of genetic variability in fish hatchery populations was resulted from some unreasonable factors in propagation process to a great extent and avoiding or minimizing these factors intentionally would be useful for maximizing the genetic variability in hatchery populations of cultural fish species.