On May 24-29,2019,a bloom occurring in Pingtan coastal areas of Fujian Province caused mass mortality of cage-cultured fish(Plectorhinchus cinctus and Pagrosomus major).During the bloom,two major causative organisms were present: Prorocentrum donghaiense(at a concentration of 1.46×107 cells/L)and an unknown naked dinoflagellate(4.58×10 6 cells/L).The naked dinoflagellate was isolated and cultured in this study,and its morphological features were examined using light microscopy and scanning electron microscopy.The large subunit(LSU)of the rRNA gene and the intal transcribed spacer(ITS)region of the naked dinoflagellate were also sequenced for fi eld bloom samples and lab culture strains(PT-A and PT-B).On the basis of its morphological characteristics and molecular sequences,the unknown naked dinoflagellate was identified as Karlodinium digitatum.According to the phylogenetic analysis,the Karl.digitatum was most closely related to Karlodinium australe and Karlodinium armiger,and the three species clustered into a single clade of Karlodinium with bootstrap/posterior probability values of 95％/0.99 and 86％/0.99 inferred from LSU and ITS sequences,respectively.Karl.digitatum was first reported as Karenia digitata,a new harmful algal species bloomed in Hong Kong,China,in 1998.In present study,we gave a detailed morphological and phylogenetic description of Karl.digitatum and submitted the molecular sequences of this species to GenBank for the fi rst time.