Condition-specific competition is a phenomenon by which inter-specific competitive dominance changes as a result of environment, and is an important factor determining species distribution. Congeneric charrs in Hokkaido, Japan, provide one of the best examples of condition-specific competition: Dolly Varden, Salvelinus malma, often dominate in cold streams (6–8°C), whereas white-spotted charr, Salvelinus leucomaenis, dominate in warmer streams (> 10°C). While past laboratory and field experiments have demonstrated the great advantage of white-spotted charr at higher water temperatures, the advantages of Dolly Varden at lower temperature have not always been clear. Here, we examined the effect of water temperature (6°C vs. 12°C) on the swimming ability of the two sympatric charrs using a stamina tunnel. At 6°C, the swimming ability of Dolly Varden was greater than that of white-spotted charr, but no difference was observed at 12°C. These results suggest that the temperature-mediated swimming ability differs between these species, which may explain the coexistence of the closely related species within heterogeneous habitats via condition-specific competition.