The acute toxicity and effects of diazinon on some haematological parameters of kutum (Rutilus frisii kutum, Kamensky, 1901) weighing 613.33 ± 157.06 g were studied under static water quality conditions at 15 °C ± 2 ºC. These experiments were carried out based on the standard TRC, 1984 method over 4 days, and controlled for the effective water physicochemical factors having pH rangesof 7-8.2, total hardness 300 mgl^-1 (caco₃), dissolved oxygen 7 mgl^-1. The results showed that the 96h LC₅₀ value of diazinon was 0.4 mgl^-1 and that the maximum allowable concentration (MAC) value of this toxin was 0.04 mgl^-1. The second stage of experiments consisted of four treatments: LC₀: 0 as blank, treatment A with a concentration of LC₁: 0.107 mgl^-1, treatment B with a concentration of LC₅: 0.157 mgl^-1, treatment C with a concentration of MAC value: 0.04 mgl^-1. Male brood stocks of R. frisii were treated with these concentrations for 45 days. The results showed that long-term exposure to diazinon causes a decrease in the erythrocyte count (RBC), haemoglobin (Hb), haematocrit (PCV), mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), leucocyte count (WBC), lymphocyte, testosterone, iron (Fe), sodium (Na), lactate dehydrogenase (LDH) and cholinesterase (CHeS) (P<0.05). Diazinon also causes an increase in prolymphocyte, aspartate aminotransferase (AST), cholesterol, alkaline phosphatase (ALP) and adrenaline (P<0.05). There are no significant effects on monocyte, eosinophil, magnesium (Mg), chloride (Cl), glucose (BS), urea (BUN), uric acid (U.A) , triglyceride (TG), calcium (Ca), albumin (Alb), total protein (TP), cortisol , noradrenaline and high density lipoprotein (HDL) levels in R. frisii male brood stocks (P>0.05). The results showed that long-term exposure to low concentrations of diazinon causes changes in some haematological and biochemical parameters of R. frisii male brood stocks .