The present study was conducted to find the histopathological variations in fingerlings of Oreochromis niloticus and Ctenopharyngodon idella after exposure to different test concentrations of fenitrothion for a period of 96 hour. The whole acute toxicity trials was carried out at constant water temperature 25°C, hardness (230 mg/L)and pH (7.5). Fishes were divided into four groups control group I, treated group II, III and IV. The treated groups were induced with sub-lethal concentrations of 3mg/L (3/10), 4mg/L (5/10) and 5mg/L (9/10) of fenitrothion concentrations for 96 hours. Sampling of liver and gills were done at  intervals of 24, 48, 72 and 96  for histopathological examination. Histological examination of the liver of Oreochromis niloticus and Ctenopharyngodon idella revealed congested sinusoids and central veins. The hepatocytes showed vacuolar degeneration. No evidence of granuloma or malignancy was seen. In addition histological examination of the gills revealed presence of lamellar epithelium hyperplasia, lamellar fusion, telangiectasis, vacuolations and lamellar blood congestion. Inflammatory cells infiltration with a minimal congestion in primary lamellae was observed. After the exposure, morphological changes appear including fusion of fins, necrosis and lesion on skin. In conclusion Fenitrothion is highly toxic and cause histopathological changes in  liver and gills  of both fishes i.e Oreochromis niloticus and Ctenopharyngodon idella.

Key words:  Fenitrothion, Oreochromis niloticus, Ctenopharyngodon idella, Gills, Liver, Histopathology.