The toxicity effect of paraquat dichloride on fingerlings of Oreochromis niloticus was investigated. The fishes were allowed to acclimatize in the laboratory for 2 week. Range finding test was carried prior to final test. At the definitive test the fishes were exposed to lethal concentrations of 0.00mg/L, 27.60mg/L, 33.12mg/L, 38.64mg/L,44.16mg/L and 49.64mg/L of paraquat dichloride for 96 hours following renewal bioassay. The toxicity bioassay showed that the 96 hours LC₅₀ was 40.768 mg/l. Behavioral responses which include restlessness, erratic swimming, loss of equilibrium, discolouration, and sudden fish death were observed in the exposed fish and these varied greatly with differences in concentrations of the toxicant. Mortality increases with increase in concentration. The differences observed in the mortalities of Oreochromis niloticus at varying concentrations were significant (p< 0.05), an indication that mortality could be a factor of concentration and time of exposure. The physico-chemical parameters of the water used for the experiment were within the tolerated limit. Based on the findings of this study there is the need to control the use of paraquat based herbicides close to aquatic ecosystems.