Necrotic spot of muskmelon, a virus disease new to Japan, was found in Shizuoka and a few other Prefectures. Three types of leaf symtoms are recognized: (I) necrotic specks, appearing in abundance on later developing leaves after infection, (II) large necrotic spots, and (III) veinal necrosis. Type II and type III symptoms are usually seen on mature leaves. On the stem and petioles, necrotic streaks are produced. On mechanical inoculation, the virus incited primary local lesions and systemic necrotic spots on muskmelon, cantaloup, and makuwa (Cucumis melo var. makuwa). It produced local lesions, but no systemic infection, on watermelon, cucumber (Chicago Pickling), and oriental pickling melon (Cucumis melo var. conomon). It did not infect other cucurbits, involving squash, white gourd (Benincasa hispida), bottle gourd (Lagenaria siceraria var. hispida), bonnet gourd (Luffa cylindrica), and oriental varieties of cucumber, and 22 species of non-cucurbitaceous plants. The virus was seed-transmitted in a moderate percentage (about 20%). A test for transmission by Aphis gossypii was negative. Test with beetles has not yet been made. The virus in expressed sap was not inactivated in 10 minutes at 60°C, during 9-10 days aging in vitro, or at 1:50, 000 dilution. Purified preparation of the virus showed, by electron microscopy, spherical particles of about 31mμ in diameter (Saito, Kishi, and Iwata, 1962). Among the known viruses infective to cucurbits, wild cucumber mosaic virus described by Freitag (1952) and Lindberg et al. (1956) seems to be most closely related to the present virus, though there are slight differences in symptoms and host range.