In breeding industries,a challenging problem is how to keep genetic diversity over ensuing generations,with the reason that a sufficient level of genetic variability is essential to maintain a sustained response from longterm selection for important traits.To investigate how mass selection process affects the genetic properties in our successive selection strains for fast growth of the Pacific oyster(Crassostrea gigas),ten polymorphic microsatellite loci were used to examine the genetic variation within population and differentiation among two wild populations,one base stock and three successive mass selection lines of C.gigas.High levels of genetic diversity of C.gigasin six populations were detected,as evidenced by large numbers of alleles per locus(N=24.0-29.7),and high levels of heterozygosities(H_e=0.925-0.956,H_o=0.724-0.809).Comp ared with the wild and base populations of C.gigas ,the mean allelic richness(R_s)in the three selectively bred generations declined slightly,but genetic heterozygosities were similar.Among the 60 population-locus cases(6 populations× 10 loci),47 cases deviated from HardyWeinberg equilibrium(P<0.05).F_is values ranged from 0.152 to 0.233,resulting in heterozygote deficiencies at the ten loci in each population.F_is values ranged from 0.008 to 0.025,showing weak genetic differentiation among the populations.The results obtained in this study show that high genetic variation exists in the three generations of selective populations,and suggest that there is still potential for increased gains in future generations of C.gigas.