The roles of algal biomass as a organic pollutant in the fresh water lakes was estimated and oxygen demand exerted by dead algal cells was determined. The results are as follows: Mean chlorophyll-a content of ATCC 4 strains, Anabaena, Scenedesmus, Microcystis, Cblorella, was 0.84% of dry weight and mean COD demanded by 1mg dry weight of algal biomass was 0.34mg. We could calculate algal COD (mg/1) from chlorophyll-a concentration (mg/m^3) in the lake water by multiplication by 0.04. The proportion of algal COD to the lake water COD was about 6.7% when the lake was in the normal state. But the proportion was increased by more than 30% during the season of algal bloom, accordingly algae contribute as a important organic pollutant in this season. BOD_5 and BOD_12 demanded by dead 1mg algal biomass of Anabaena flos-aquae were 1.3mg, 1.6mg respectively. With this respect algal biomass can be serve as a serious organic pollutant and can consume much oxygen in the lake especially during the death phase of algal bloom.