Abstract: There is an argument whether the bloom causative species of the East China Sea is Prorocentrum dentatum or a new species. Based on the scanning electron-microscopic study, we compared the detailed surface structure of the sample from the East China Sea and Prorocentrum dentatum strain (CCMP1517) from the Provasoli-Guillard National Center for Culture of Marine Phytoplankton, USA. The results showed that these two samples belonged to one species. After reviewing literatures,it can concluded that the bloom causative species of the East China Sea is Prorocentrum dentatum, rather than a new species.