A melanocytic tumor was found in the skin or subcutaneous tissue in a BDF₁ mouse maintained up to 109 weeks. The tumor was solid and well-demarcated from the surrounding tissue, and showed an extensive metastasis to lymph node, liver, and lung. The tumor was composed of both oval or polygonal cells with abundant melanin pigments, and spindle cells mixed with the oval or polygonal cells. These blackish pigments in the tumor cells disappeared by bleaching and more blackened by Fontana-Masson stain. The tumor cell showed poor cellular pleomorphism and rare mitosis. Numerous mature, stage IV-melanosomes were observed in the cytoplasms of the tumor cells by electron-microscopical examination. It was suggested that the origin of the present tumor was a dermal melanocyte by its histological features, though no definite conclusion was obtainable because of ulceration in the central area of the skin covering the nodule.