Liver tumors were observed in 2 South American lungfish (Lepidosiren paradoxa) and in 1 African lungfish (Protopterus amphibius) kept in 3 different aquariums in Japan for 2-5 years. In the first case a single, large nodule (70 X 60 X 55 mm) in the liver had been noted externally as a swelling 1 year prior to death. In the second case 2 rounded nodules (25 X 15 X 15 mm and 15 X 10 X 10 mm) were incidentally found at autopsy. In the third case, which also demonstrated abdominal swelling for 1 year preceding death, 2 green nodules (50 X 50 X 40 mm and 17 X 17 X 17 mm) were found in the liver together with multiple metastatic lesions. Histologically, variation was evident from tumor to tumor, even within the same animal. The most common histologic type was typical trabecular hepatomas. Admixtures of glandular or papillary structures were noted in some tumors. A region of poorly differentiated hepatocellular carcinoma with spindle-shaped cells in a sheetlike arrangement was noted in one case. The cause of these tumors is unknown. Since liver tumors were discovered at relatively high incidence (3/14 autopsied cases, including 6 species), lungfish seem to be predisposed to the development of liver tumors.