The determination of toxic compounds present in the. aquatic and its effect on fish is a basic environmenf. issue in aquatic. toxicology (Bolis et al., 2001). Preferred way to evaluate the ecological influence of toxic compounds is. mortality or bioassay (Svobodava et al., 2003). In this work, a member of organophosphorus pesticide (malathion) and European chub fish (Leuciscus cephalus) were selected for the bioassay experiment.