This paper deals with the studies on the occurence of a new virus disease of sesame and the identification of the causal virus. The virus disease of sesame has been regarded as a widespread disease in the sesame-growing areas in the southern part of Korea. The disease was found to be caused by watermelon mosaic virus (WMV). During the years since 1978, stunting of sesame plants, with yellow mosaic, necrotic spot, and malformation, were collected from 17 different places. Virus isolates from 27 out of 32 samples were identified as WMV. Natural infection of squash, pumpkin, cucumber, and watermelon by WMV as well as sesame was proved. The virus is inactivated at temperatures of 55 to $60^{circ}C$, at dilution of $10^{-3};to;10^{-4}$, and in the aging of 10 to 14 days at about $20^{circ}C$. Sesame, Chenopodium amaranticelor, pea, bean, as well as many plants of the Cucurbitaceae, are susceptible to the sesame-isolates of WMV. In negatively stained preparations, particles of the virus appear under the electron microscope as flexible filaments of about $750~800nm$ in length. Cylindrical inclusions and virus particles were found in the cytoplasm of mesophyll cells by ultra-thin sections of WMV infected tissues.