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The behavioral and biochemical responses of Japanese medaka (Oryzias latipes) to acute and subacute (sublethal) levels of chlorpyrifos were studied. In the acute exposure test, medaka were exposed to 0.018, 0.055, 0.166, or 0.500 mg L(-1) chlorpyrifos for 4 d. As a result, fish showed hypoactivity compared to the control (at 0.018, 0.055, and 0.166 mg L(-1), swimming speeds were 55.6%, 39.0%, and 27.3% those of the control), Brain acetylcholinesterase activity and swimming speed were significantly correlated. In the subacute toxicity test, medaka were exposed to 0.012 mg L(-1) chlorpyrifos (10% of LC(50)) for 8 d. On day 4, there were no significant differences in behavioral and biochemical endpoints in exposed fish as compared to the control. On day 8, exposed fish became hyperactive, and the swimming speed of the social group increased to 2 times that of the control, whereas acetylcholinesterase activity was decreased to 68% that of the control. In addition, fish exhibited significant alterations in social behavior (schooling duration increased to 2.6 times and solitary duration decreased to 28% that of the control). Our findings clearly demonstrate a subacute effect of chlorpyrifos on the social behavior of medaka, which may pose a risk at population level because of the disturbance of social behavior. In addition, the recorded behavioral alterations may provide a useful tool for assessing the toxicity of organophosphorous pesticides to aquatic organisms., (Copyright © 2013 Elsevier Ltd. All rights reserved.)