The histologic and ultrastructural features of hepatic hemangiopericytoma from a medaka (Oryzias latipes) exposed for 48 hr to 400 mg/liter of diethylnitrosamine at 14 days of age are described. The predominant histologic pattern was of spindle-shaped cells forming numerous whorls around central capillaries, vacuolated areas, or necrotic debris. The predominant cell type was a spindle-shaped cell with oval nuclei, elongated cell processes, and abundant organelles converging upon normal appearing capillaries. Occasionally, however, they converged upon cells swollen with cytoplasmic filaments and/or containing large fenestrated or debris-filled cytoplasmic vacuoles. These features were reminiscent of endothelial cells undergoing intracellular canalization seen in angiogenesis or neovascularization. Individual capillaries were also seen in the mass independent of whorls. It was not clear, as is the case in man, if capillary formation was an integral part of the neoplastic process or a reactive response. Although the liver is an unusual location for hemangiopericytoma in man, many of the cellular features in the fish tumor were similar to the human tumor. The ultrastructural characterization of tumor cells in fish carcinogenesis correlated with histologic patterns of growth will expand our understanding of how fish cells respond when transformed, and augment the development and use of aquatic bioassays for carcinogenesis research.