Stomach contents of the largemouth bass collected from Lakes Shorenji (typical reservoir) and Nishinoko (typical swamp) were analyzed to determine its feeding habits and environmental adaptability. Main preys were a goby Rhinogobius sp(p). and shrimps Macrobranchium nipponense and Palaemon paucidense in Lake Shorenji, and a crayfish Procambarus clarkii and a shrimp M. nipponense in Lake Nishinoko. Fishes and crustaceans were the most important prey in Lakes Shorenji and Nishinoko, respectively. Principal preys changed ontogeneitically from the goby into bluegill Lepomis macrochirus and a cyprinid fish Tribolodon hakonensis in Lake Shorenji, and from a shrimp M. nipponense to the crayfish in Lake Nishinoko. The feeding activity was intense in summer and declined in winter. The bass may affect the ayu Plecoglossus altivelis altivelis population in both lakes. The bass has very flexible feeding habits, and this allows it to adapt easily to various feeding environments. The bass in Lake Nishinoko fed on larger amounts of prey organisms than that in Lake Shorenji. This fact may cause the higher growth rate of the bass in the former lake.