Abstract: Both Prymnesium sahans and Prymnesium parvum can secrete toxins and cause fish mortality. There was controversy on whether it was Prymnesium sahans or Pryrnnesium parvum that caused fish mortality in China. Prymnesium parvum has been examined by electron-microscopy; some reports suggest that the pattern of scales provides grounds for classification. So far, Prymnesium saltans has not been observed by etectronmicroscopy. It is difficult to differentiate between Prymnesium saltans and Prymnesium parvum. Since 1984, we have systematically studied the peculiar mode of swimming, body form, cyst, propagation, pattern of the scales, and structure and function of the flagella of Prymnesium saltans by light and electronmicroscopy and also the biology and other aspects of this species. We have proved that it was Prymnesium saltans instead of P. parvum that caused fish mortality in TianJin region. It has also been proved that the scale can be regarded as one of the characteristics of the genus, but not of the species.