The objectives of the study were to analyze chemical water quality and fish community structures in the downstreams of Geum River during 2004~2005. Water quality parameters showed that ionic dilution was evident during July~August due to a rainfall and river runoff by Asian monsoon. Mean concentrations of BOD showed a seasonal variation and were greater in the downstream than in the midstream. The total number of fish species observed was 34 (11 families) and 30.3% of them was Korean endemic species. The dominant species with > 20% of the total were Zacco platypus (25.9%) and Microphysogobio jeoni (21.5%) in the river. Exotic species observed was only one species, Carassius cuvieri with relative abundance of 1.8%. According to the structure analysis of fish community, species richness index was highest (3.714) in S6 and lowest in S4 (1.961), while species diversity index was highest in S2 (1.01) and lowest in S5 (0.507). Tolerant species dominated the fish community (49%), and the sensitive species were rare (4.4%), indicating a biological degradation of ecosystem. In the mean time, omnivore species was composed of 49% in the fish community and insectivore species was nearly same with the proportion of the omnivores.