In the present study, the effect of methyl parathion (MP), an extensively used organophosphorus insecticide in agriculture, on liver apoptosis in Chalcalburnus tarichi, an endemic carp species of the Lake Van Basin of Turkey, was investigated. For this purpose, fish were exposed to sublethal concentrations (1.47, 3.0, and 6.11 mg/L) of MP for 30 days under semistatic conditions. Following exposure, sections taken from the liver were stained using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling method for the examination of apoptosis, and then apoptotic hepatocytes were evaluated as the apoptotic index (AI). According to the results, the AI was significantly enhanced at MP concentrations of 3.0 and 6.11 mg/L. In conclusion, the present study indicates that MP has apoptosis-inducing potential in the liver of C. tarichi and MP-dependent hepatotoxicity causally related to the increase in the apoptosis