Abstract: In order to study the pollut ion of potent liver cancer promoter, microcystins (MCs), in water supply reservoirs and some typical lakes in Guangdong province, water samples from 12 reservoirs and 4 lakes were collected in summer, autumn and winter in 2003, and in 7 reservoirs and 1 lake b-i monthly in 2004 to analyzeMCs by the method of high performance liquid chromatography (HPLC) and enzyme-linked immunosorbent assays (ELSIA).The results showed that MCs distributed widely in freshwaters in Guangdong province, and were detected almost in all water bodies.MC-RR was the main type of MCs in freshwaters of Guangdong Province.Concentrations of MCs ranged from ND to 0.919Lg/L in reservoir water samples, and high contents of MCs were observed in water samples from Tangxi reservoir, Hedi reservoir, Qiyeshi reservoir and Shenzhen reservoir.The city park lakes such as Liuhua Lake in Guangzhou city and Xihu Lake in Zhaoqing city were polluted byMCs as well, and the highest concentrations were 0.640Lg/L and 0.559Lg/L, respectively.MCs concentrations increased a lot during 2004 survey compared to 2003 survey.The result suggested that the water supply reservoirs in Guangdong province have been contaminated by MCs to some extent, and efficient ways should be taken to prevent the further deterioration of water quality from water supply reso