Abstract: Cyprinus carpio, a common species of fish in Chinese freshwaters, was selected as subject in the study on eco-toxicological effects of MC-LR, atrazine, and their combination on the fish. Results show that the coexistence of MC-LR and atrazine, two non-base toxicants, at relatively low concentration could induce significant histopathological alterations in the livers and gills. The mixture of 1 μg•L^-1 MC-LR and 5 μg•L^-1 atrazine would cause to the fish histopathological damages, such as ruptured epithelium of the central retinal vein, acini pancreas, hemolysis, hepatocyte vacuolation, pyknosis, etc. in liver of the fish. The symptoms were more apparent and serious than those in the fish exposed to single MC-LR or atrazine. Electron microscopic scanning of the gill tissues show that the histopathological damages induced to the gill by MC-LR combined with atrazaine were more severe: especially the gill lamellae array became loose and messy, boundary of the epithelial cells blurred and the cells severely broken. Furthermore, the combined exposure to 5 μg•L^-1 MC-LR and 5 μg•L^-1 atrazine could induce a significant hepatic apoptosis of C. carpio. The findings indicate potential risks of the coexistence of different pollutants, even at low concentration, in the water environment to aquatic organisms, and provide valuable references for determination of safety threshold of pollutants and perfection of aquatic ecological risk assessment of pollutants.