The impact of short-term exposure to waterborne chlorpyrifos-ethyl on Clarias gariepinus was evaluated through changes of selected behavioural parameters. Fish was exposed to 0.64 mg/l, 0.80 mg/l, 0.96 mg/l, 0.12 mg/l, 0.28 mg/l and control for 96h. The parameters measured were Opercular ventilation (OVC), Tail fin movement rate (TMR), Air gulping index (AGI) and Mortality. The main effects of concentration and duration on rates of tail fin movement rate of fish were highly significant

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... . There was significant decrease (p<0.05) in the opercular ventilation of treated fish when compared with control. Also both the main effects of concentration and duration of exposure of chlorpyrifosethyl and the effect of their interactions on AGI were highly significant (p<0.001). Highest mortality was recorded from 1-12hrs period of exposure and the mortality at 12hrs was significantly (P<0.05) higher than the mortality recorded from 24hrs-96hrs periods. The acute toxicity of chlorpyrifos-ethyl elicited dose and duration dependent behavioural changes that led to mortality of fish. Behavioural changes were therefore proven to be more sensitive endpoints than mortality.