Abstract: Allozyme variation within three natural populat ions of Crassostrea plicatula from Shandong coastal waters ( LZ, LD and QD ) and one cultured population of C. gigas from Rongcheng ( TP) were investigated by horizontal starch gel electrophoresis. 20 loci were detected in 9 enzymes. The percentages of polymorphic loci were 60% in LZ and QD populations and 55% in the other two. High variation was shown in LZ and QD populations. Mean heterozygosity observed in LZ, LD, TP and QD was 0. 216 ? 0. 050, 0. 165 ? 0. 036, 0. 139 ? 0. 034 and 0. 195 ? 0. 042 respect ively. Mean effect ive number of alleles varied from 1. 207 ? 0. 086 to 1. 422 ? 0. 165. More genetic variations were shown in LZ and QD populations than in LD and TP populations. Heterozygous deficiency was common in all populations. Nei distance between TP and the other three populations of C. plicatula were very similar to each other, but the largest one still below that of LZ and QD ( 0. 0500) . The smallest Nei distance was between LD and TP populations( 0. 0099) . The research indicated that C. plicatulaand C. gigas might belong to the same species of C. gigas .