Lesser-spotted leatherjacket Thamnaconus hypargyreus is an economically important fish species in the South China Sea.It is necessary to study its genetic structure for the exploitation and management of the fishery resources.Fish specimens(totally 156 individuals)used in this study were collected from 5 sampling locations in the north of the South China Sea and the southwest of Nansha.The results were as follows:the mitochondrial cytochrome b(Cyt b)gene sequences of the fish was amplified by polymerase chain reaction(PCR)technology.Both 56 mutations of nucleotide acids without inserting or deleting one and 58 haplotypes were found among the examined 779 base-pair nucleotide sequences of Cyt b fragment.The ranges and the level of average genetic distance were 0.002 95-0.004 15 for T.hypargyreus between groups.All 5 groups were characterized with high haplotype diversity(0.820 1-0.980 4)and relatively low nucleotide diversity(0.002 62-0.004 69).Analysis of molecular variance(AMOVA)and the fixation indices(F_st)of 5 groups showed that the genetic variance mainly came from individuals within groups,and there was no genetic differentiation between groups.With high gene flow,this group could be developed and managed as an evolutional significant unit.The median-joining(MJ)network and phylogenetic tree proved no phylogeographic differentiation structure in 5 groups.According to the criteria distinguishing different populations,species and genera given by scholar Kartavtsev,the five groups should be merged to the same population,which implied T.hypargyreus in the North and South-West Continental Shelf of the South China Sea belongs to one population.The findings would provide a scientific basis for the study of migration of T.hypargyreus between the two Continental Shelves and management of its stock.