Histopathological effects of different sublethal concentrations (1, 2 and 4 mg/l) of diazinon, an organophosphorus pesticide, on gill, liver, spleen and kidney of grass carp (Ctenopharyngodon idella) were studied after 1, 7, 15, 30 and 45 days post-exposing fishes weighting 850 ± 155 g to the toxicant for 12 hours at 18-22ºC. In addition, tissues of nostril were examined for transmission electron microscopy and scanning electron microscopy. Results of light microscopic examinations of tissues showed that toxicant was causing severe histopathological effects such as congestion of blood vessels, haemorrhage, cellular infiltration, pyknosis of cells nuclei, vacuolar degeneration and general necrosis in the kidney, spleen and liver. There were also degenerative changes of interstitial tissue and detachment of tubular basement membrane in kidney observed. Hyperplasia and fusion of secondary lamellae, separation and sloughing of epithelium from the underlying basement membrane were also observed in gill sections. transmission electron microscopy (TEM) examination of nostril revealed, damages of epithelial cells (vacuolated club cells and abnormality in cells' nuclei). scanning electron microscopy (SEM) examination of nostril showed a reduction in excretion of amorphous proteinous materials, reduction in vesicle numbers on the cell surface and also closing of cell surface canals.