Static bioassays were made to determine acute toxicity of premium motor spirit on poecilia reticulata. Lethal doses of the oil were determined using LC50 software programmes based on Finneys probit statistical method. The 96 LC50 value of the oil for guppy was estimated as 1.69 (1.13-2.43) mg L-1. The fish exhibited behavioural changes in the form of aggression, rapid gulping of water, increased opercular movement and abnormal swimming movements. The significance of this sublethal effect particularly as a stress indicator of petrol oil contamination was discussed.