Chattonella marina, a raphidophycean flagellate, is one of the most noxious red tide phytoplankton and is highly toxic to fish, especially to yellowtail, Seriola quinqueradiata. Blooming of Chattonella has repeatedly caused severe damage to fish farming in Japan. One of characteristic features of this flagellate is the production of reactive oxygen species (ROS) such as superoxide anion (O ^2- ) and H₂O₂ under normal growth conditions. Since harmful effects of ROS have been well documented in various biological systems, ROS may be a responsible factor for fish mortalities by Chattonella. Although the detailed mechanism of ROS generation is still unclear, we have recently found that lectins such as concanavalin A (Con A), wheat germ agglutinin (WGA), and castor bean hemagglutinin (CBH) stimulate C. marina to generate increased amounts of O^2- , suggesting the presence of signal transduction pathway leading to O^2- generation similar to the oxidative burst in phagocytic leukocytes in mammals. In cell-free extracts prepared from C. marina cells, NAD(P)H-dependent O^2- generation was observed, and this response was blocked by diphenyleneiodonium (DPI), a potent inhibitor of mammalian NADPH oxidase. When the cell-free extract of C. marina was analyzed by immuno blotting using antibody raised against the human neutrophil cytochrome b558 large subunit (90 KDa), a major component of NADPH oxidase, several immunoreactive proteins were detected and one of them was approximately 110 kDa that was slightly larger than human cytochrome b558. These results suggest that C. marina have a plasma membrane enzyme system analogous to the neutrophil NADPH oxidase as a source of O^2- production.