Abstract: Using morphological taxonomy,we identified a freshwater dinoflagellate from the Freshwater Algae Culture Collection(strain No.FACHB-329) as Peridinium umbonatum var.inaequale Lemmermann by morphological taxonomy.We also described the morphology of its cysts.We sequenced the small and large subunit ribosomal DNA(SSU and LSU rDNA) for morphological identification.The phylogenetic results indicate that Peridinium as presently defined is polyphyletic and the taxa of our studied genus could be divided into two large branches: a clade of Peridinium umbonatum group and another clade of Peridinium sensu stricto group.Both of the morphology and molecular datas indicate that there are differences between P.umbonatum group and Peridinium sensu stricto group.Different phylogenetic results support P.umbonatum group may cluster into a monophyletic freshwater group in spite of the low bootstrap values,while the monophyletic Peridinium sensu stricto group had high bootstrap values.Freshwater P.aciculiferum and P.wierzejskii are very closely related to marine Scrippsiella.