This study was carried out to investigate phytoplankton distributions and water properties at 6 sampling places in Yongsan lake during the period from Jun. 30 to Nov. 17, 1991.Results obtained were as follows: 1. At sampled sites, BOD value ranges from 1.0~14.0 mg/l except Sangmongtan where it was 14.0 mg/l on Nov. 17. 2. T-N and T-P were generally high and the former was 0.26~1.42 mg/l, the latter 0.010~0.158mg/l. 3. At all sampled places, salinity was in the range of 0.04~0.57% but especially downstream of Yongsan lake was high, 0.39~0.57% on Oct. 27. 4. The phytoplankton identification in this survey period showed Cyanophyceae is 11 genera 14 species, Bacillariophyceae 13 genera 22 species, Chlorophyceae 30 genera 55 species: total 54 genera 91 species. 5. Dominant species were Anacystis aeruginosa, Lyngbya contorta, Anabeana Sp, Melosira granulata, Melosira granulata var. angustissima, Dictyosphaerium pulchellurn and Actinastrurn hantzschii var. fiuviatile during the period from summer to fall. 6. Lyngbya contorta was seemed to adapt to the circumstance of low transparency. 7. Phytoplankton distributions were seemed to be influenced by salinity and thransparency. And so they might appear much fewer in the downstream than the upperstream in Yongsan lake.