Bacteria play a major role in removal of pollutants in activated sludge processes, but the reason why bacterial population changes is not yet clearly understood. This study focused on the chemicals produced by microorganisms in activated sludge. Chemical components in an activated sludge from a laboratory sequencing batch reactor (Lab SBR) was extracted with ethanol, and activated sludges from two sources, Lab SBR and a wastewater treatment plant (WWTP) were incubated with and without the extracted chemicals. Incubation was performed on a microplate which has 96 wells and each of the well is regarded as a small activated sludge reactor. After 24 hours incubation, activated sludge samples were recovered, and their microbial populations were analyzed by reverse transcription PCR (RT-PCR) targeting a partial 16S rRNA gene sequence followed by pyrosequencing. The results were preliminary, but clearly showed that bacterial population changed significantly when activated sludge was incubated with ethanol-extract addition. In addition, activated sludge from Lab SBR and WWTP were similarly affected by the extract.