The objective of this study was to investigate the relation between water quality and the species composition of microorganism in the stream purification facilities. The results showed that the concentration of dissolved organic matters like dissolved organic nitrogen (DON), dissolved protein decreased as the wastewater flowed down. In August, the concentration of the DON was very little (0.19-0.43 mg.l_-1), among the three points the seed biofilm was fetched out, population of the bacteria raised in the media with inorganic nitrogen source was higher that with the organic ones. However, in February, media with organic nitrogen source led to higher bacteria population than those with inorganic nitrogen source inthe case that the seed biofilm was stripped out from the upstream point. On theother hand, the biofilm sampled from the middle-and down-stream area, the media with inorganic source resulted in higher bacterial population than the ones of organic source. In the upstream point where the organic concentration was higher, zoomastigophara was dominant protozoa. In February, the ciliate Colpidium sp. was the dominant species with the its population occupied 88-95% of the total protozoan population. It suggest that the occurrence of different dominant species should be the different characteristics of the bacteria assimilating the nitrogen. It was clarified that the microbial composition of the biofilm was affected bythe nitorogen composition and its concentration