A virus was isolated from soybean plants showing yellow vein symptoms that were collected in a field of an experimental station located in the central part of Thailand in 1982. The virus was a new virus of soybean based on particle morphology, host range, stability in crude sap, and so on. It was designated as "Soybean yellow vein virus" (SYVV). The occurrence and distribution of SYVV disease were limited to a field where soybean was cultivated in rotation with sorghum. Numerous rod-shaped virus particles (about 500-550 nm in length and 15-20 nm in width) were detected in the dip preparations of soybean leaves infected with SYVV. Chenopodium amaranticolor and C. quinoa showed distrinct chlorotic local lesions on the inoculated leaves, and only soybean was recognized as a systemic infection host of SYVV. Aphis gluycines (0/60) and Bemisia tabaci (5/50) failed to transmit SYVV. The dilution end point, thermal inactivation point and longevity in vitro of SYVV were, 10^-3-10^-4, 35-40°C (10 min) and 2-3 hours at 4°C respectively. Attempts to obtain a purified preparation of SYVV were made. After sucrose density gradient centrifugation, the peak fraction identified in the relative absorbance curve with ultra-violet light (254 nm absorbance zone) showed the highest infectivity and also contained numerous rod-shaped particles.