Abstract: Toxic strains of Prymnesium parvum were isolated from fish ponds with blooms of a toxigenic flagellate P. parvum and high fish mortality. The pure culture was successfully achieved in laboratory. Culture of P. parvum grew well in medium of artificial sea water medium (Salinity, 12-16‰) and natural sea water at 23t and 600-800 lx illumination. Haemolytic activity appeared during the stationary phase of the exponential growth phase and maximized at the latter stationary phase. Two kinds of toxins, i. e. hemolysin and ichthyotoxin were extracted from Prymnesium cells as well as from extracellular medium. The activity of P. parvum haemolysin and ichthyotoxin was detected respectively using bovine erythrocytes and Poecila reticulala. The dependence of P. parvum ichthyotoxin on various co-factors was also studied. The major component of a homolysin was identified being a glycolipid.