Diazinon is commonly used for pest control in the agricultural fields surrounding freshwater reservoirs. This study was conducted to determine the chronic toxicity of organophosphorous pesticide and its effects on some hematological parameters and biochemical blood plasma profiles of common carp, Cyprinus carpio. Diazinon was applied at concentrations of 60 and 120Î¼g/L preparations in 10, 20 and 30 days since the experiments were initiated. The experimental groups showed significantly lower values (p < 0.05) of erythrocyte count, haemoglobin content, haematocrit, leucocytes, Lymphocyte and monocyte, as well as in alkaline phosphatases and significantly higher (p<0.05) values of plasma glucose, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, neutrophile and eosinophile compared to the control group. There was not significant difference in value of total protein (p< 0.05) in the experimental groups and control group. Values of MCV, MCH and MCHC of experimental species were compared to the control groups. The results of examinations of the biochemical blood plasma profile indicate a marked neurotoxic effect of diazinon in fishes. Changes in values of both erythrocyte and leukocyte profile after exposure to diazinon-based preparation may be referred to disruption of haematopoiesis as well as to a decrease on non-specific immunity of the fish.