Freshwater fish populations in an irrigation pond, Seta Tsukinowa Ooike, were examined inOctober and November from 1984 to 1995 using the mark-and-recapture method. Fishes werecaptured by bait traps and angling, marked by fin clipping, and recaptured one or two weeks later. An indigenous cyprinid, topmouth gudgeon, Pseudrasbora parva, was abundant in 1984 but decreased abruptly after 1985 , possibly due to the population increase of bluegill. Population of an exotic fish, bluegill, Lepomis macrockirus, fluctuated at an interval of 2 - 4 years. The largest abundance of 0+ bluegill was observed in 1987, which was nearly equal to the abundance of topmouth gudgeon in 1984. The largest abundance of large bluegill was observed in 1985 and 1987, being about one third of that of 0+ bluegill. An increase of 0+ bluegill was observed when the population of large bluegill (more than 1+) had declined. Population of another exoticpiscivorous fish, largemouth bass, Micropterus salmoides, increased after 1992. In 1995, this level was one fifth of the largest abundance of large bluegill, and probably caused a decline of the bluegill population.