Microcystis is one of the most common and most studied bloom-forming cyanobacteria and a well-known producer of microcystins. Different cladocerans usually exhibited different responses to the toxicity of Microcystis. In the present study, the effects of toxic Microcystis from Lake Taihu were investigated by performing, acute tests on cladocerans (Daphnia longispina, Daphnia carinata, Diaphanosoma brachyurum and Ceriodaphnia cornuta), using crude Microcystis extract. On a diet of Microcysits (1 10⁶ cells/ml), the median lethal time(LT₅₀) of four cladocerans were 3.03、3.79、8.48 and 9.25 days, respectively. The longer LTS0 were recorded for D. brachyurum and C. cornuta which have small size. Among the experiment, there was no new neonates of D. longispina and D. carinata, and few new neonates of D. brachyurum and C. cornuta. The 24h-LC₅₀ of D. longispina, D. carinata, D. brachyurum and C. cornuta were 4.10×10⁶、4.78×10⁶、6.21×10⁶和7.77×10⁶ cells/ml. So, the effects of Microcystis on the survivorship and fecundity of cladoceran population in Lake Taihu were very intense, and the effects of D. brachyurum and C. cornuta which have small size were relatively gently.