This study was carried out to show the histopathological alteratious induced in the hepdtopancreas of the crayfish Procambarus clarkil exposed to the insecticide, diazinon. The hepatopancreas is normally formed of tubules that are composed of digestive and secretory cells. The digestive cells contain vacuoles and some of them have yellowish brown granules that probably represent residual bodies of lysosomes, Exposure of crayfishes to increasing concentrations of diazinon has led to numerous pathological alteiations in the hepatopancreatic cells especially the digestive cells. The tubules appear widely separated by connective tissue, while the cells are more vaculated and characterized by loss of cytoplasmic density and presence of numerous yellow granules. The degeneration of the cells was more pronounced at 1/2 LC50 / day for 4 days of diazinon. The crayfishes exposed to sublethal levels for long period showed slight changes in the hepatopancreas. This indicates that the alterations are dependent on both the insecticide concentration and
the exposure time.