Seasonal changes in body size and stable isotope ratios of bluegill Lepomis macrochirus were monitored at three sites in Lake Biwa in an attempt to investigate the latitudinal variation in the fish growth pattern. δ13C values of bluegill tended to decrease as latitude increased, implying the presence of a thermal gradient across the lake. In contrast, δ15N values of bluegill increased with increasing latitude. Better digestive performance of the fish at higher latitudes was assumed to explain this result, whereas the discharge from a sewage treatment plant seemed to have a more direct impact on the extremely high δ15N values of bluegill at the highest latitude site. Length—weight relationships were used to assess the changes in the body weight of fish with a normalized body size. Bluegill at higher latitudes exhibited greater body weights with a prolonged duration of body weight gain. Bluegill at lower latitudes may have adapted to grow rapidly from late spring to early summer when feeding activity increases in response to increasing water temperature, while the catabolic activity remains at a low level. In contrast, bluegill at higher latitudes can continue to grow until the midsummer months owing to the lower water temperature. The seasonal growth pattern of bluegill was analyzed using a logistic-type equation. The assumption that the selection of growth rate or final body size depends on temperature may reasonably explain the latitudinal variation in the growth pattern of bluegill in Lake Biwa.