The effects of smectite suspended in seawater on the blood properties of the Japanese flounder (Paralichthys olivaceus) were examined. The fish were exposed to smectite (0, 4.5, 10, 18 and 56 g/l) seawater solutions at 20°C. All the fish died within 4.5 h exposure to a 56 g/l smectite solution, and almost all fish within 24 h exposure to a 18 g/l smectite solution. Blood was collected from the dead fish soon after death and from the live fish after 24 h exposure, and was chemically analyzed. Hemoglobin and plasma protein concentrations did not vary significantly after exposure to the smectite solutions. Compared to the live fish and the control fish, the dead fish showed markedly high plasma lactate, blood total ammonia, and plasma electrolyte (Na⁺, Cl^-, K⁺, Ca²⁺, Mg²⁺) concentrations and significantly low plasma glucose concentrations. Taken together, changes in the blood properties revealed that the lethal physiological disturbances were attributable to hypoxia caused by exposure to the high concentrations of smectite.