This study was achieved to study the effects of atrazine on some selected blood hematological, serum components and associated histopathological studies in catfish. Investigations of acute and long term toxicity of atrazine in catfish (Clarias gariepinus) were carried out. Acute toxicity was applied in a semi-static test during a 96-hr exposition. Mortality was recorded after 24, 48, 72 and 96 hrs. Lethal concentration inducing 50% mortality (LC-50) was calculated. Long term toxicity was investigated on 70 Catfish divided randomly into two groups of 35 each .The first group was kept as control (normal water), the second was exposed to 4.5 mg/L atrazine for four weeks. Sampling was carried out at 0, 7, 14 and 28 days. Biochemical, hematological and histopathological changes in certain organs and tissues were investigated. The results showed that, the estimated LC-50 value was 18.1 mg/1. Hematological results showed significant decrease of RBCs, hemoglobin and haematocrit value and changes of MCV, MCH and MCHC as compared with the control groups. Biochemical changes revealed significant reduction in levels of glucose, total protein, cholesterol, total lipids and triglycerides than the control ones .Most prominent enzymatic changes were in the alkaline phosphatase, transaminases, and acetylcholine activities whereas the most severe histopathological changes were observed in the fish gills followed by liver, kidney and intestine. With increase of the time of exposure to atrazine, gill lesions and other lesions of the different organs increased in severity.