Fishes are very sensitive to the changes in their aquatic environment. For this reason, they are known as the bio-indicator species to monitor the water pollution. Organophosphate pesticides are widely used amidst various group of pesticide in intensive agricultural practices to protect the crops from various pest and diseases owing to their high insecticidal property, low mammalian toxicity, low persistence and rapid biodegradability in the ecosystem. Pesticide exposure may also fatal to many non-target organisms like fish where it hampers its health through impairment of metabolism, occasionally leading to the death of the fish. Malathion (C10H19O6PS2), one of the earliest organophosphate insecticides is being extensively used as dust, emulsion, and vapour to control wide variety of insect pests under different conditions. Malathion, one of the most extensively studied pesticides, may induce many significant changes in fish. Present study is aimed to review the toxicological effects on haematological parameters, physical parameters, biochemical parameters, behavioural changes, neurotoxic, histopathological alterations, respiratory responses, bioaccumulation and chromosomal changesin fishes exposed to the organophosphate pesticide Malathion.