Since fish population is a basic unit of study and management of fishery resources,analysis of fish population is a foundation for biological and ecological study on fish reproduction,distribution and migration.Using molecular biology method,the paper analyzed population of Saurida tumbil,an important commercial species from northern continental shelf waters of South China Sea. A total of 78 samples of Saurida tumbil were collected from Beihai,Zhanjiang,Maoming,Dongguan,and Shanwei of northern South China Sea.We found there are 48 haplotypes and 39 variation sites in 568 bp gene fragments of 78 sequences obtained by sequencing the mitochondrial D-loop of the samples.Analysis of molecular variance(AMOVA)indicates that the genetic variability within groups is 98.82% and that among groups is only 1.18%.The genetic differential index(Fst)and the genetic distance between every two groups are 0.000 24-0.030 08 and 0-0.025,respectively.Molecular tree constructed by NJ method and the network constructed by TCS1.21 show that the individuals of different groups are not clustered as one independent group and intermixed together,which indicates that there clearly exists a great gene flow among groups,and the genetic diversities among groups are at quite lower levels.According to the criteria distinguishing different populations,species and genera given by scholars Theorpe and Shaklle,the five groups should be merged to the same population.However,Saurida tumbil population built its geological groups by means of continuously rapid growth in small ones after undergoing fast expansion,differentiation and bottleneck phase,which not only accords with the traditional viewpoint of basic characteristics of fishery resources of South China Sea to a certain extent—“The populations of most species belong to local population without a longdistance migration”,but also is regarded as an evidence of molecular biology.