This study was carried out bimonthly from October, 1989 to August, 1990. As a part of preliminary ecological survey to elucidate the struture and function of lower Mankyong River ecosystem. Water temperature, pH, salinity and inorganic nurients (ammonia-N, nitrite-N, nitrate-N and phosphate-P) were measured. Annual variation of total heterotrophic bacteria, physiologically characteristic bacteria and fecal coliform bacteria were also measured for microbiological analysis. When we considered the parameters in all its aspects, water quality near Mokchon Bridge (St. 4) was remarkably polluted as compared with other stations. Total heterotrophic bacterial population during investigation periods was ranged from 5.0 x 10³ c.f.u. ml to 1.3 x 10^6 c.f.u. ml. Seasonal mean values of proteolytic bacterial populations showed higher densities than those of lipolytic and amylolytic bacteria at all stations. Fecal pollution near St. 4 was more serious than other stations. Among 93 isolates, 55.9% were rods and 64.5% were Gram negative. According to morphological, physiological, nutritional and biochemical tests, isolated bacteria adapted not only broad range of water temperature, pH, and salinity but also various kinds of nutrients.