Massive abdominal enlargement was observed at relatively high incidence in two species of fish kept in two public aquariums for several years. Three seminomas, one dysgerminoma and two fibromas were found in 14 largemouth bass, Micropteras salmoides, kept in a public aquarium for over 80 months. Four seminomas and one nephroblastoma were found in 24 Japanese dace (ugui), Tribolodon hakonensis, kept for more than 57 months in two different aquariums. The seminomas in M. salmoides showed rapid growth, and the affected fish died within a few months. At necropsy, a single large tumor, measuring 10.5-12.0cm, was found in the abdominal cavity. The seminomas in T. hakonensis were single or multiple tumors, measuring 2.0-4.5cm in diameter. Histologically, these seminomas were composed mainly of a typical germ cells similar to those in human seminomas or embryonal carcinomas. A nephroblastoma in one T. hakonensis showed extensive metastases to various organs. The cause of these tumors is unknown, but the prolonged longevity of fish kept under artificial conditions may have enhanced their development.