Abstract: The population genetic variation and phylogeographical patterns for 88 individuals of Zacco platypus from 9 drainage systems in Guangdong region were investigated preliminarily based on nucleotide sequences of mtDNA cytochrome b gene.The complete cytochrome b gene sequences were detected to be 1140 base pairs,with 98 variable sites,75 parsimony informative sites and 33 haplotypes.Only one haplotype was found in the population of the Janjiang River,while multiple haplotypes were shown in the populations of the rest 8 drainages.Hap11(BJ4_BL3_LX3_LD2_JJ1) were shared by the five populations in the Beijiang River,the Liuxi River,Luoding River,the Beiliu River and Jianjiang River,Hap4 (LD1_BL1) were shared by the populations in the Luoding River and the Beiliu River,Hap6 (DJ1_LX2) were shared by the populations in the Liuxi River and the Dongjiang River.Hap29 (HJ4_RJ4) were shared by the populations in the Hanjiang River and the Rongjiang River.It showed a high genetic diversity,for the average value of haplotype diversity (h) and nucleotide diversity (π) were 0.908 and 0.01961,respectively.Phylogenetic tree estimated by neighbor-joining (NJ) method indicated that 33 haplotypes of Zacco platypus populations were divided into two lineages.Lineage A contained the populations in both the Pearl River drainage systems (including the Beijiang River,the Dongjiang River,the Liuxi River,the Luoding River and the Beiliu River) and the Western Guangdong drainage systems (in-cluding the Jianjiang River and the Moyang River).The populations in the Eastern Guangdong drainage systems (including the Hanjiang River and the Rongjiang River) were composed of Lineage B.The genetic distance with the range from 0.0517 to 0.0549 and the percentage divergence of sequences with the scope from 5.35% and 6.49% between these two lineages were relatively higher than those within Lineage A (0.0012-0.0099,0.26%-2.11%)and Lineage B(0.0027,1.58%).However,the value of the genetic distance and the percentage divergence of sequences between these two lineages were much lower than those between Zacco platypus and outgroups (0.0945-0.1912,8.77%-17.11%).The result showed that there were a huge genetic differentiation between Lineage A and B,while the differentiation degree did not approach to the level of Species differentiation.Owning to the geographical obstruction of the Lianhua Mountains,the populations from the Hanjiang River and the Rongjiang River were relatively independent.According to the haplotype network,this fish from the middle and west regions in Guangdong Province had probably originated from the Liuxi River,and spread to the Xijiang River,the Beijiang River,and the Dongjiang River of the Pearl River drainage systems respectively,then continued to spread to the Jianjiang River and the Moyang River.In addition,the fish can also spread from the center,through the Dongjiang River to the Rongjiang River and the Hanjiang River.Analysis of molecular variance (AMOVA) revealed that genetic variation among the populations within regions was 38.50%,and that within the populations was 66.24%.Tests of neutrality and mismatch distribution suggested the whole population of Zacco platypus from 9 drainage systems in Guangdong region were relatively stable,without obvious population expansion.