The possible effects of the chlorpyrifos (CPF) on the fish Cyprinus carpio L. exposed to sub-lethal concentrations (52, 79, and 158 mu g/L) of CPF for 6 weeks were evaluated, and an analysis was made of their hematological parameters, biochemical parameters, and histology of various organs such as liver, gill, and muscle. At three concentrations, packed cell volume (PCV), mean corpuscular hemoglobin, Red blood cells (RBC) Hb, T3, T4, and total protein were significantly decreased in fish treated with CPF, whereas the parameters White blood cells (WBC), creatinine, Glutamic Oxaloacetic Transaminase (GOT) and Glutamic Pyruvic Transaminase (GPT) presented a significant increase at the two higher concentrations. However, the sub-chronic exposure to CPF resulted in histological lesions and caused clear damage to liver, muscles, and gill tissues of the fish Cyprinus carpio. Thus, we may conclude that the altered biochemical and hematological parameters can be used as efficient biomarkers in monitoring the toxicity of CPF in aquatic organisms. At the same time, histopathology proves as a reliable and easy tool for toxicological studies.