The effects of acidification (acidic proceeding) on the physico-chemical factors and bacterial population in Wangsong reservoir, Kyonggi-Do were studied in batch culture microcosm system during the summer of 1995. The numbers of total bacteria, hetero-trophic bacteria and functional groups of bacteria were ranged from 0.71 to 2.42×10^6cells/ml and from 0.001×10^4 to 9.33×10^5cfu/ml at various pH values in batch culture system, respectively. Acid stress had no apparent effect on total bacteria, in contrast heterotrophic bacteria and functional groups of bacteria showed response to pH changes. The mean percentage of heterotrophic bacteria and functional groups of bacteria among total bacteria were 16.53% and below about 1%, respectively. Heterotrophic bacteria and functional groups of bacteria showed a positive correlation with pH, turbidity, and dissolved oxygen, whereas it showed significant negative correlation with ammonia, Al, Mn, Zn, and Cu.