Abstract: Lake Biwa consists of a large, deep north basin and a small, shallow south basin, and the body weights of bluegill Lepomis macrochirus in the north basin tend to exceed those in the south basin. To elucidate the cause of this phenomenon, the physical characteristics of the fish in the two basins were compared. The fish in the north basin had larger gonads and stomachs than did the fish in the south basin, but the contributions of these organs to the body weight were very small. The body weights of the fish in the north basin still exceeded those in the south basin after the subtraction of the weights of the gonad and stomach. Bluegill in the north basin had deeper bodies than those in the south basin. The heavier body tissues of bluegill in the north basin appear to be an adaptive response to the colder environment. Given the flourishing of largemouth bass Micropterus salmoides in the littoral zone in the north basin, the deeper bodies of bluegill there may contribute to their increased survival rate by reducing their vulnerability to predation by largemouth bass.