The study of toxic effects on ecosystem level is ecotoxicology. In this review ill effects of commonly used toxicants: dimethoate and malathion on fish are highlighted. These chemicals are organophosphate insecticide introduced in 1960 which contributed greatly to pest control and agricultural output. The main use of pesticides in India is for cotton crops (45%), followed by paddy & wheat. Liberal use of these chemicals played havoc with human and other life forms. From treated plants and soils these pesticides reach surface water through runoff, irrigation and rain, thus contaminate aquatic ecosystem. These chemicals get their entry in to food chain & found to be highly toxic to aquatic organisms mainly fish. The contaminated water causes various ill effects on fish health.