Abstract Fasinex (triclabendazole) has been reported to be an active fasciolocidal agent used in humans and in farm animals. The clastogenic effects of fasinex were tested in lymphocyte cultures of the river buffalo at three final concentrations: 25, 50 and 100 μg/ml. Chromosomal aberrations, sister chromatid exchanges and micronucleus formation are the three cytogenetic parameters used in this study. The results demonstrated that the number of cells with different types of chromosomal aberrations, including chromatid breaks and gaps, isochromatid breaks and gaps and polyploidy, was increased significantly in cultures treated with different doses of fasinex compared to the control. This increase was dose-dependent where there was a positive correlation between increased drug concentration and induction of chromosomal aberrations. The frequency of sister chromatid exchanges and the formation of micronuclei in all lymphocyte cultures treated with different doses of fasinex were increased significantly compared to the control; these increases were also dose-dependent. In conclusion, the three cytogenetic parameters used to evaluate the effect of fasinex revealed that the drug has a strong clastogenic effect on river buffalo lymphocytes in vitro.