The present research aimed to determine acute toxicity and evaluate the effect of sub-lethal concentrations of diazinon on some haematological and immunological parameters of Barbus sharpeyi by the Organization for Economic Cooperation and Development methods. Juvenile B. sharpeyi were exposed to 0, 5%, 10% and 25% 96h LC₅₀ concentrations of diazinon for 21 days. Blood samples were taken after 7, 14 and 21 days of exposure to diazinon. Haematological and biochemical parameters including: RBC, Hb, PCV and WBC, as well as serum enzymes were evaluated. Immunological indices and resistance against bacterial infection, were determined. Results showed that 96h LC50 of diazinon in B. sharpeyi was estimated at 3.987 mg/L. Diazinon toxicity at a level of 1 and 0.4 mg/L decreased RBC, Hb, and Hematocrite, in almost all sampling periods compared. WBC and globular index decreased significantly in fish exposed to 1 mg/L diazinon on days 14 and 21(p<0.05). Dose dependent increase in serum enzymes were seen in fish exposed to diazinon. Diazinon toxicity showed no effect on serum LDH level (p>0.05). Dose dependent decrease in serum total protein and globulin were indicated in diazinon exposed fish. Serum lysozyme and bactericidal activity decreased in T3 and T4 at days 14 and 21. Mortality following challenge to A.hydrophila increased in fish exposed to 1 mg/l diazinon. It can be concluded that diazinon is toxic to B. sharpeyi. Therefore, the strict biosecurity should be taken into consideration when this pesticide is used in agricultural fields surrounding freshwater sources of fish cultivation