A fundamental goal of ecotoxicology and hazard assessment is determining the ecological effects of toxic chemicals on natural communities and ecosystems. Little information exists regarding the toxicity of pendimethalin to fish. Pendimethalin is a dinitroaniline herbicide used to control the spread of many annual grasses and broad-leaved weeds. Because of the widespread use of pendimethalin in many fields, it has been detected as a contaminant in groundwater and is now known to be one of the most common aquatic contaminants.There has been increasing interest in the use of fish(oreochromus niloticus) as biomarkers for the effects of pollution for the early detection of aquatic environmental problems especially the development of industrial, anthropogenic, and agricultural activities as leading to increase of contaminants in aquatic ecosystems and adversely affect human health.