The research was designed to determine the behavioural responses and acute toxicity of different concentrations of Ampicillin to Clarias gariepinus Juveniles in an indoor experiment. The lethal examination lasted for 96 hours using 0.00mg/L, 10.00mg/L, 20.00mg/L and 30.00mg/L respectively in a rectangular glass tanks, aerated and stocked at fifteen fish per tank. The behavioural, physical, mortality and water parameters were observed and LC 50 determined using probit value. The results indicated that fish exposed to ampicillin showed agitated swimming, air gulping, loss of equilibrium and haemorrhage within the 96hours exposure. The mortality recorded increased according to the increase in the exposure duration and concentrations. The LC 50 =13.80mg/L and safety level=1.38mg/L while a prediction equation of Y = 0.0783 + 4.3294X, R 2 = 0.993 p<0.05 was recorded for Ampicillin. Ampicillin is considered more toxic than all the three antibiotics and should be administered with caution.