The ichthyofauna and fish community structure at the downstream of the Hyongsan River were investigated from April 1991 to March 1992. The ichthyofauna was composed of 37 species and 31 genera belonging to 15 families. Of which three species, S. multimaculatus, R. uyekii and C. longicorpus were endemic species in Korea and two species, H. molitrix and L. macrochirus were transplanted fishes. The component species were markedly different between brackish and fresh water areas: in brackish water area 42.3% of collected fish were sea fishes but in fresh water area 75% were primary freshwater fishes. The diadromous fishes were 9 species; of which 5 species; Lateolabrax japonicus, Mugil cephalus, Tridentiger obscurus, Rhinogobius brunneus and Acanthogobius flavimanus were amphidromous, 3 species; Hypomesus olidus, Tribolodon hakonensis and Gasterosteus aculeatus were anadromous and one species; Anguilla japonica is catadromous. The dominant species were C. auratus, P. parva, P. sinensis kaibarae and M. oxycephalus in brackish water area. The rare species less than 1% of all were 14 species. As a whole, the dominance index was 1.113, diversity index 1.143 and evenness index 0.729. The dominance index was somewhat higher than that of other rivers in southern part of Korean peninsula but the community structure were rather stable and diverse.