An unusual malignant fibrous tumor of the testicular tunic in a 35-year-old Japanese man is presented. It apparently arose from the visceral layer of the tunica vaginalis where focal mesothelial hyperplasia was seen. Microscopically the tumor conststed of interlacing bundles of spindle cells and numerous polygonal cells with variable amounts of collagen fibers. The electron microscopic study demonstrated the former cell type closely resembling the fibro-blast and the latter to be more epithelial having very plump cytoplasm with fairly well developed desmosomes. The tumor was diagnosed as malignant fibrous mesothelioma upon histopathologic and ultrastructural comparison with the similar tumors in the pleura and peritoneum. Because of the scarcity of the diagnosis in the region it is suggested that fibrous paratesticular tumors are critically reviewed.