A fungus was isolated from the regions of dermatomycosis on Japanese pond turtle, Mauremys japonica. The fungus was tentatively identified as Aphanomyces sp.of Saprolegniaceae. However, we could not define it at species level because the fungus did not produce any syngenetic organs. The optimal growth temperature for Aphanomyces sp. strain AK11 was estimated as 25-30°C. This is the first report of Aphanomyces sp. infection in Japanese pond turtle.