Abstract: In China, lake eutrophication is serious. Many lakes are close to death because receiving too big amounts of polluted water. Recently, heavy blooms frequently occured in Dianchi Lake and Microcystis is the dominant species in the bloom, and in many cases, when appeared in the lake, forms a continuous bloom all along the year, eliminating quite all the other species. Microcystis produces a family of related cyclic hepatopeptides(microcystins,MC). These toxins are severely hepatotoxic, produced in Microcystis cells and released into water body when algal cells were broken. Most investigations about the toxicity of the microcystins are focused on animals and higher plants. To our knowledge, few studies have been made on the possible effects of microcystins on algae, which are equally important as primary producers in the whole ecosystem. In this study, We chose the unicellular Synechococcus elongatus(one of the most studied, and geographically most distributed, cyanobacteria in the picoplankton) as test material and investigated the toxicological effects of MC-RR on it. For this purpose, some physio-biochemical parameters (cell optical density, chlorophyll fluorescence(Fv/Fm), pigment content(chlorophyll、phycocyanin, soluable protein content, soluable and insoluable carbohydrate content) were tested in algal cells when exposed to 100μg/L microcystin-RR. Microcstin-RR was isolated and purified with the high performance liquid chromatography(HPLC) method. Thereafter, known amount of purified toxin in distilled water was added to test culture flasks in the way to obtain a final toxin concentration in cultures of 100μg·L-1. The results showed that the growth of Synechococcus elongatus(shown as optical density) was significantly inhibited compared with the controls. After exposure to the toxin, the OD665 were lower than those of the controls from day 2 to day 10. The OD665 of the control kept in a increasing tendency but that of the toxin -treated decreased on day 8, which meant that the algae had came into death phase in advance. At the same time, a difference in chlorophyll flurorescence(the efficiency of excitation capture by open photo system Ⅱ(PS Ⅱ)reaction centers, expressed as Fv/Fm) also appeared. Fv/Fm of the controls kept at a stable high level. In constrast, that of the toxin-treated algae decreased quickly from day 4. The results also demonstrated that toxin treatment could cause the changes of some biochemical characters. For example, the content of soluable protein and soluable, insoluable carbohydrate all reduced after toxin exposure. Alough the content of chlorophyll and phycocyanin of the toxin-treated algae decreased, the ratio of PC/Chl was higher than that of the control. From th present study ,we could infer the possible toxicological mechanism of microcystin-RR on the cyanobacterium Synechococcus elongatus. Microcystin could inhibit the photosynthetic ability and the inhibition may cause the lack of nutrition(protein,carbohydrate),the decrease of the biochemical reactions and the cell division rate. As a result ,the growth of the algae was significantly influenced. These results also suggested that microcystins could cause lethal effect on competitive algae and microcystins maybe play a role in phytoplankton succession and population regulation.