BACKGROUND: Diazinon is an organophosphate pesticide which is widely used in paddy fields located in northern parts of Iran, though it is prohibited to be used in many countries. OBJECTIVES: The purpose of the present study was to investigate sublethal effects of diazinon on blood parameters of common bream Abramis brama. METHODS: In sublethal toxicity test, fish were exposed to diazinon concentrations of 0.04, 0.36, 0.73 and 1.46 mg/l for 14 days and their hematological parameters including WBCs, white cell differential count, RBCs, Hct, Hb, MCV, MCH and MCHC were studied.  RESULTS: The results of sublethal toxicity indicated that by increase of toxicant concentration a significant decrease was appeared in WBCs, RBCs, Hct, Hb and MCH both after 7 and 14 days (p<0.05). After 7 days a significant decrease was observed in lymphocytes count which accounted for about %81 of the control group. The neutrophils count was also face the same reduction so that after 14 days of diazinon exposure, the neutrophils counted decreased to about %85 of control group. CONCLUSIONS: Regarding the toxicity of diazinon on various hematological parameters and the diazinon concentration in Iranian waters and considering the fish habitat in the waters which are connected to the Caspian Sea it suggests that the pesticide diazinon affect the fish survival.