Objectives of this study were to understand the relationship between mortality and abnormal behavioral responses, biochemical parameters for cypermethrin. Juveniles of Oreochromis niloticus were exposed in aquaria of 20 litres capacity containing 0.007, 0.008, 0.009, 0.010 and 0.011 mg/L nominal concentrations of cypermethrin for 96 hours to determine acute toxicity in a static bioassay. The behavioral and morphological responses observed include loss of equilibrium, hyperactive to hypoactive, over-reactive to under reactive to startle response, and posturing of pectoral fin. The 96h LC50 for the fish was determined using probit analysis and found to be 0.0126 mg/L. Biochemical analysis of the blood of fish examined showed that TG, TP, GOT, GPT, and cholesterol levels decreased significantly (P≤ 0.001) with increase concentration of the pesticide while triglycerides level increased with increase in concentration of the pesticide. It was concluded that cypermethrin is poisonous and had a significant effect on the behaviour and biochemical system, which adversely affected the health of the fish.