1. To understand fish population dynamics in the Choon Chun Impoundment, the Uiam Impoundment and the Achim Reservoir, the author studied the species, interspecific composition ratio and age composition of each species collected in the three reservoirs and in the North Han River System over a 4 year period from 1967 to 1970. 2. Each reservoir has distintive characteristics, the Choon Chun Impoundment and the Uiam Impoundment are resorvoirs dammed for electric power and the Achim Reservoir is an irrigation reservoir stored water in flood season. The maximum sarface areas, and maximum depths, of the Choon Chun, and Uiam Impoundments and the Achim Reservoir are 1,432ha -32.5m, 1.720ha -19m and 17ha -13m respectively. 3. Species number of the fish collected in North Han River, the Choon Chun Impoundment, the Uiam Impoundment and in the Achim Reservoir are 54, 26, 40 and 13 respectively. 4. The dominant species was Zacco platypus in the three reservoirs. However, the species which are over five percent in individual numbers, are, in order of their numbers, Opsariichthys bidens, Siniperca scherzeri, Gobius similis, Hemibarbus labeo, Pungtungia herzi, Microphysogobio koreensis, Pelteobagrus fluvidraco and Coreoperca herzi in the Choon Chun Impoundment; Opsariichthys bidens, Pelteobagrus fluvidraco, Siniperca scherzeri, Hemibarbus labeo and Gobius similis in the Uiam Impoundment; Carassius carassius, Opsariichthys bidens and Hemibarbus longirostris in the Achim Reservoir. 5. Each species of the fish in each reservoir is composed of several age group. Young fish were generally more abundant than old fish. The author also found much varation in size even in the same age group.