This study is dealt with the effects of toxic Microcystis aeruginosa and Microcystin (MCYST) on fishes and Daphnia magna by feeding and intraperitoneal injection. The results suggested were as follows:1. MCYST-LR was pernicious to all of the tested fishes. 1) Grass Carp:mean weight was about 10-12 g. The dosage of injection was about 65-300 μg/kg, LD₅₀ 110 μg/kg; 2) Silver Carp:mean weight 18-20 g, dosage of injection 150-400 μg/kg, LD₅₀ 350 Hg/kg; 3) Tilapia:mean weight 32-35 g, dosage of injection 400-800 μg/kg, LD₅₀ 500 μg/mg. Liver injury was discovered by observing fish liver altrastructure 24hrs after the injection with Microcystins. The liver damage of Grass Carp was more serious than that of the other two fishes. 2. When Silver Carp and Tilapia fed on toxic M. aeruginosa as the only diet, they can be alive for more than 20 days. The toxicity of feces decreased 2-4.5 times respectively compared with that of M. aeruginosa. The absorption spectra and microscope observation of the feces of both fishes reflected that the ability of digesting and absorbing of toxic algae cell and toxin for Tilapia was greater than that of Silver Carp; both fishes had little liver damage by observing their liver ultrastructure, 3. Fed on Micnx'ystis ueruginosa or added with 15 μg/mL MCYST-LR to the medium, Daphnia magna may die 48hrs later. Daphnia magna grew slowly and did not carry eggs when fed on nontoxic M. aeruginosa 8508, showing that Microcystis was not a suitable food for Daph-nia.