The Razor Clam (Sinonovacula constricta)is an important shellfish in China,Japan and Korea. We sequenced mitochondrial cytochrome oxidase Ⅰ ( CO Ⅰ) genes of 197 individuals from nine populations to investigate population genetic structure of the northern (ZH,HG,HY),the mid-regional(YC,DT,BZ) and the southern(NB,TZ,ND)groups of S. constricta along the coast of China. We identified 125 haplotypes and 96 variable sites from 556 bp of CO Ⅰ. Nucleotide diversity ranged from 2. 176 4 to 7. 497 0. Analysis of molecular variance ( AMOVA) showed that interpopulation variance accounted for 80. 27% ( P < 0. 05 ), whereas the remaining 18. 74% and 0. 99% were detected within population and among populations of same groups respectively,indicating significant genetic differentiation among populations. Phylogenetic tree analysis and genetic distance analysis showed that the nine populations were clustered into three distinct groups,with the northern group clustering first with the southern group and then with the mid-regional group.