A study of the cytological and ultrastructural characteristics, chromosomal value and tumorigenesis of a cultivated golden hamster (Mesocricetus auratus) malignant melanoma was made. Four kinds of cells were observed in the cultures: melanocytes, fusiform cells and two forms of macrophages. Electron microscope observations showed very irregular nuclei with intranuclear lesions, in a type of ring‐like bodies. Vast Golgi zones with melanin formation were seen in the cytoplasm. The melanosomes presented a laminar formation. Extracellular virus‐like particles, 80 mμ in diameter, were observed occasionally. The modal value of chromosome numbers was 44 and they ranged from 42 to 270. When inoculated in hamsters, as small as 500 cells/ml developed tumors in 44–60 days with metastases which retained the property to produce melanin. The significance of the nuclear lesions and virus‐like particles in the golden hamster melanoma cells remain open for di