Vertical polyacrylamide gel electrophoresis was employed to investigate the genetic variation of isozyme of Charybdis japonica. Four geographic populations in Dalian Black Reefs (DL), Shangdong Laizhou Bay (LZ), Jiangsu Haizhou Bay (HZ) and Zhejiang Xiangshan (XS) (48 individuals each) were surveyed. Ten enzymes revealed twenty-eight putative loci, in which nine were polymorphic. For the four populations, the proportion of polymorphic loci (P0.99) was 32.14%. The mean effective number (Ae) of alleles per locus ranged from 1.283 0 to 1.297 9, and the mean values of observed (Ho) and expected (He) heterozygosities were in the range of 0.146 2~0.155 6 and 0.273 8~0.278 3 respectively. The genetic distance (Dnei ) among the four wild geographic populations ranged from 0.000 4 to 0.001 8, which were lower than that between subspecies. Cluster analysis indicated that DL and LZ populations were clustered together for the closest relationship, while HZ or XS, respectively, was an independent cluster.