The whitefly-borne mungbean yellow mosaic virus (MYMV) in Thailand was transmitted by mechanical inoculation. Among the several buffers used in attempted transmissions, 0.1 M potassium or sodium phosphate, pH 7.8, gave the highest transmission rates. The optimal incubation temperatures for symptom expression ranged from 25 to 30°C in the growth chamber or 30°C in the daytime and 20°c at night in the greenhouse. Host range of MYMV was limited to seven plant species in the family Leguminosae. Determinations of the stability of the virus in plant sap gave the following results : thermal inactivation point of 40-50°C for 10 min, dilution end point of 10^-2-10^-3, and longevity in vitro of 1- 2 days at 20°C. Purified virus preparations had an ultraviolet light absorption spectrum typical of that of nucleoprotein with a A₂₆₀/ A₂₈₀ value of about 1.3-1.4. Purified preparations and leafdip samples contained geminate particles about 18 x 30 nm in size. Infectivity was associated with the presence of purified virus particles. In thin sections, the virus particles were isometric, about 15-20 nm in diameter, and they often formed loose aggregates that sometimes almost filled completely the nucleus of infected phloem cells. Mungbean infected by whitefly transmission or by grafting showed hypertrophied nucleoli, aggregates of virus particles, and fibrillar bodies in the nuclei of phloem cells as early as 2 days before symptom appearance. In vacuoles or lumens of the partially or fully differentiated infected sieve elements, virus particles occasionally formed aggregates having a double cylindrical arrangement of particles. No virus particles were detected in tissues other than the phloem of infected plants or in any tissues of comparable healthy plants.