Abstract: Nodularin is a new kind of cyanobacterial toxins found in water blooms worldwide, and fish is very easily to suffer from the negative effects induced by this toxin. This study found that nodularin could induce the apoptosis of Ctenopharyngodon idellus lymphocytes in vitro. The observation of transmission electron microscopy showed that under the impacts of nodularin, the lymphocytes presented typical apoptosis features, such as obvious cytoplasm condensation and nuclear chromatin agglutination and marginalization. The DNA ladder fragmentation further confirmed the occurrence of the apoptosis of the lymphocytes. After incubated with 1, 10, and 100 μg·L^-1 of nodularin for 12 h, the percentages of apoptotic lymphocytes reached 19.4%, 31.6%, and 71.6%, respectively, suggesting a dosedependent manner. The nodularininduced apoptosis was related to the increase of intracellular reactive oxygen species (ROS), decline of mitochondrial transmembrane electric potential, upregulation of intracellular Ca²⁺, downregulation of Bcl₂, and upregulation of Bax. Meanwhile, the activation of Caspase-3 and Caspase-9 were involved in the process of apoptosis. These results strongly indicated that nodularin could induce the apoptosis of fish lymphocytes via mitochondria pathway.