To clarify the mechanisms by which the yellowtail Seriola quinqueradiata are killed by the red tide plankton Chattonella, an analysis was made on physiological responses of the fish exposed to the plankton. Fish were chronically cannulated in the dorsal aorta and gill cavity. Changes in respiratory, ionic and cardiovascular parameters were followed until death. Soon after exposure, a severe hypoxemia developed which was accompanied by a marked increase in ventilatory pulse pressure, though the latter subsequently subsided. No acidosis occurred until shortly before death despite the persistent hypoxemia. Plasma concentrations of Na⁺, K⁺, Cl^-, Mg²⁺, and Ca²⁺ were all elevated to different magnitudes. Blood pressure responses varied among fish, but arrhythmia was the predominant response. Heart rate gradually declined or remained unchanged until shortly before death. Gill ventilatory frequency changed very little during most of the exposure period. These results were critically discussed in the light of data reported previously.