Starry flounder (Platichthys stellatus) and Oshoro flounder, hybrids of starry flounder and stone flounder (Kareius bicolaratus), collected in the brackish-water Lake Kuccharo, Hokkaido, were examined for skin pseudotumors. The disease was found in Oshoro flounder, which represents the first record of skin pseudotumors from pleuronectid hybrids. In starry flounder, only small fish (less than 10 cm in body length) had skin pseudotumors, and prevalence of the disease sharply decreased with fish size. No predatory fish was caught in the lake. It thus seems likely that small starry flounder die due to skin pseudotumors in the lake.