Acute toxicity (96 h LC50 – 0.7 ppm) of chlorpyrifos was evaluated in the freshwater fish Labeo rohita in static bioassay over a 96–hours exposure period using probit method. Histopathological investigations revealed various degrees of pathological lesions in gill. The gill showed the fusion, malformation of secondary lamellae at the tips, vaculation, and necrosis and detached of gill epithelium. The histological alterations in the fish could be used as an important tool for assessment of aquatic pollution.