Hepatic tumors were induced in medakas (Oryzias latipes) by the addition of diethylnitrosamine (DEN) to their aquarium water. Medakas are useful for study because they are highly susceptible to the carcinogenic effect of DEN and the time for tumor induction is relatively short. Tumors had a histological pattern similar to that of mammalian hepatic tumors. The medaka seems to be susceptible to water‐soluble carcinogens.