Massive abdominal enlargement was discovered in 8 adult carp (Cyprinus carpio) 4-6 years old, living in breeding ponds of carp fisheries in northern Japan. The abdominal enlargements rapidly progressed and the affected fish died within a few months. At necropsy, single or multiple tumors up to 20 cm in diameter were found in the abdominal cavities. Ovarian tissue identified within the tumor capsule in 4 fish supported the impression, based on gross anatomic features, that the neoplasm arose in the ovaries. Histologically, the tumors were composed mainly of various types of mesenchymal cells. In one sample, striated muscle was identified by electron microscopy; in another, squamous cell nests were intermixed with mesenchymal elements. Although some tumors had areas of cellular pleomorphism suggesting a malignant character, no evidence of invasion or metastasis was found. These neoplasms were classified tentatively as dysontogenetic tumors of the ovary, possibly teratoid in nature.