during selection over four-generations for shell height using nine microsatellite loci. The mean effective number of alleles per locus () ranged from 1.435 to 2.096 among the four JCS generations. The mean allelic richness (odecreased from 0.492 to 0.269, suggesting there was a decrease in genetic diversity. In general, the allelic frequency remained stable over the four generations, but fluctuated up and down, apparently randomly, between generations. For example, the frequency of the HHM20-158 bp allele changed regularly from mid to high levels. Our results suggest that persistent selective breeding resulted in reduced genetic diversity and somewhat stable genetic structure in pearl oyster.