Grazing effects of a freshwater bivalve, Unio douglasiae, on both the phytoplankton communities of cold- and warm eutrophic lakes were examined in aquarium with sediment. The animal U. douglasiae used in the study was collected from the Gunsan (Jeonbuk), acclimatized in laboratory aquaria with washed sand from the mussel collection stream, and starved for $2{sim}3$ days by the experiment. Grazing experiments were performed with the surface waters collected from Lake Ilgam (Seoul) in the cold period (March 31, 2008), and the warm period (June 19, 2008), respectively. Results clearly showed that the concentration of chlorophyll-a (chl-a) and algal abundance were decreased with the increase of mussel-treated density and treated-time. At the same animal density, U. douglasiae effectively decreased the chl-a of cold-lake water (CW), compared to warm-lake water (WW). Increasing the mussel density, the filtering rate (or algal abundance) of the mussel on the CW gradually decreased, while those of WW did not increase or decrease. Major phytoplankton species Synedra ulna and Stephanodiscus hantzschii, and Scenedesmus ecornis in CW, and those of Tetraedron regulare and Pediastrum simplex in WW, were quickly decreased after the mussel stocking. Interestingly, cyanobacterium Microcystis aeruginosa in WW, not dominant species, were less removed. These results suggest that a freshwater bivalve U. douglasiae have a strategic potential to control dense hibernal diatom or green algae, rather than the cyanobacterial bloom in eutrophic water.