Abstract: The occurrence of heavy water blooms of cyanobacteria (blue-green algae, BGA) in surface water is receiving increasing attention world wide as a potential health concern. This is a serious water quality problem because many of the cyanobacterial species or strains are able to produce potent toxins. The most frequently reported cyanobacterial toxins are cyclic heptapeptide hepatotoxins, microcystins, isolated from several species of the freshwater genera Microcystis, Planktothrix (Oscillatoria), Anabaena and Nostoc. They are potent and specific inhibitors of the serine threonine family of protein phosphatases, especially PP1 and PP2A. In order to minimize public exposure to MCs, the World Health Organization (WHO) has set a provisional guideline value of 1 μg/L for MC-LR in drinking water and established the tolerable daily intake (TDI) for consumption of cyanobacteria products contends MCs (0.04 μg/kg/day). Aquatic animals could bioaccumulate microcystins(cyanobacteria hepatotoxins) and so, beyond water, the ingestion of contaminated food represents a human health risk. In order to evaluate the potential risk of their ingestion, this study was mainly investigated the residue levels and distribution features of microcystins in fish from eutrophicated Lake Dianchi, a hypertropic lake in Yunnan Province containing toxic cyanobacteria blooms all the year round. Samples of silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis) and grass carp (Ctenopharyngodon idellus) were collected in the biomanipulation test area in Lake Dianchi respectively in April and September, 2003. Microcystins in different tissues were all analyzed by enzyme-linked immunosorbent assay (ELISA) and the concentration was expressed as microcstin-LR equivalent. The results showed that microcystins presented in all samples, mainly distributing in liver, kidney and digestive tracts, and little in the muscle and the non-digestive tract organs. Different residue levels of microcystins were obviously in different fishes. Concentrations of microcystins in liver and kidney, the target organs of silver carp as well as bighead carp were significantly higher than that in grass carp. On the other hand, the residue levels of microcystins were also changeable along with seasons. Concentrations of microcystins in fish samples in April were lower than that in September. Finally, the safety assessment of the microcystin in fish muscle was also evaluated. According to the standard recommended by WHO, we calculated that the concentration of microcystins in fish muscle of all samples were not above the limit for human consumption. Then, we simply concluded that microcystins in fish muscle had not yet endangered to the human health only existed in the potential health risks.