The 5.8S rDNA,ITS,partial 18S rDNA and 28S rDNA sequences were amplified from two Alexandrium sp.strains(Alexandrium sp.qd2.Alexandrium sp.qd3),which were isolated from Jiaozhou bay of Qingdao,and preliminarily identified as Alexandrium sp.based on morphological features.By blastn analysis,phylogenetic tree construction and genetic distance calculation,Alexandrium.sp.qd2 and Alexandrium.sp.qd3 were identified as A.catenella,and belong to different strains of A.catenella.