Abstract: Sargassum vachellianum C.Agardh is native to China and mainly distributed in coastal waters of Zhejiang,Fujian,Guangdong and Hong Kong,China.It is important in maintaining the structure and function of littoral ecosystems and has been chosen as the main candidate for seaweed bed reconstruction by artificial breeding in China.Thus,to conserve and sustainably use this species,we should make genetic studies on it.As the traditional classification and phylogenetic relationships in Sargassum have been challenged by DNA phylogenies and the status of numerous species have been re-assessed,it is necessary to know whether the traditional taxonomy of S.vachellianum needs to be revised.However,there was no DNA markers have been reported in this important species.In this study,we cloned and sequenced ITS sequences of 15 S.vachellianum individuals from 3 different geographic populations.As a result,4 different ITS sequences were obtained.Sequence analysis revealed that the lengths of the four ITSs were all the same and their ITS1,5.8S and ITS2 were 762,158,and 507 bp,respectively.There were 3 mutations among the four ITS sequences,but no geographic population specific sites were found.The phylogenetic relationships among the four S.vachellianum ITSs and 24 GenBank ITS sequences from three genera of Sargassaceae were studied with sequences from Hizikia fusiformis,Turbinaria conoides and T.decurrens as three outgroups.The Neighbor-Joining phylogenetic tree constructed by MEGA 4.0 showed the four ITSs of S.vachellianum were clustered together first and then clustered with S.sp. and S.piluliferum,indicating a close genetic relationship corresponding to the smallest genetic distance(0.004)between S.vachellianum and S.sp.The greatest pairwise genetic distance(0.422)existed between S.vachellianum and T.conoides.