The whitefly-borne mungbean yellow mosaic virus (MYMV) observed in Thailand was transmitted by mechanical inoculation. The optimal incubation temperatures for symptom expression on plant ranged from 25 to 30°C in the growth chamber. Host range of MYMV was limited to seven plant species in the family Leguminosae. Determination of the stability of the virus in plant sap gave the following results : thermal inactivation point : 40-50°C for 10 min, dilution. end point : 10^-2-10^-3, and longevity in vitro : 1-2 days at 20°C. Purified virus preparations consisted of geminate particles about 18 x 30 nm in size with an ultraviolet light absorption spectrum typical of that of nucleoprotein (A₂₆₀/A₂₈₀ value of 1.3-1.4). Infectivity was associated with the presence of purified virus particles. In ultrathin sections, the virus particles were isometric, about 15-20 nm in diameter, and aggregates of virus particles sometimes filled almost completely the nuclei of infected phloem cells as early as 2 days before symptom appearance. Nucleic acid isolated from MYMV particles was identified as circular singlestranded DNA with a molecular weight of 8.0 x 10⁵. MYMV was assigned to the Geminivirus group.