Chlorpyrifos is an organophosphorous insecticide widely used in agriculture and domestic activities. However, little is known about the effect of this pesticide on the immune system of fish and other alterations of its physiological system. The aims of the present study were the evaluation of the LC(50) (lethal concentration(50)) and the potential toxicity of this substance on Nile tilapia (Oreochromis nilotucus), as well as its effect on some hematological values and phagocytic functions of this fish. Results obtained showed that chlorpyrifos does not have any effect on the following parameters: number of red blood cells, hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). However, the phagocytic index and the percentage of phagocytic active cells were significantly affected, which could indicate that phagocytic parameters from Nile tilapia are more sensitive than hematological ones to assess the effect of acute intoxication with chlorpyrifos.