In order to investigate the level of pollution and self-clarification activity in Daechung reservoir, seasonal and vertical variations of environmental factors, population size of heterotrophic microorganisms and activities of exoenzymes hydrolysing organic matters were analysed from January to September, 1994. Environmental factors such as pH, temperature, chssolved oxyeyen showed remarkable variation according to season and depth, especially in summer time. Thermal stratification was formed around the water depth of 5∼10m. The population size of total heterotrophic bacteria was higher at July and lipolytic bacteria were predominant throughout the year in Daechung reservoir. But amylolytic bactria became prominent at July. Fungi and yeast population have not showed significant variation according to season and depth, but there was somewhat higher fungal cells in July. There was not distinct seasonal variation of exoenzyme activities. In contrast, vertical variation was clear particulary in summer time. And the ratio of abiontic exoengyme activities were higher throughout the year except phosphatase. From the results, it is considered that there is a little external suppliment of pollution and the self-clarification activity is high in Daechung reservoir.