Seasonal changes in the incidence of Rice Yellow Orange (Tungro group) virus disease in Thailand, in relation to population and rate of viruliferous individuals of the vector,Nephotettix virescens, were studied in field experiments. The disease incidence fairly coincided with population trends of the vector. Infection was found to occur in the nursery as well as in the paddy field. Among several weeds, only wild rice, Oryza rufipogon, proved to be an alternate host of the virus. Latent period, i.e. the time needed for inoculated plants to become infectious, was found to be as short as 2 days. This, in combination with the nonpersistent nature of the virus transmission, may account for the very high incidences of the disease caused by lower population of the vector, as compared with other rice virus diseases in the temperate zone. The results of insecticide application trials, damage analysis, experiments on the effect of temperature on growth of the vector, the effect of moulting of the vector on virus retention, etc. are also described and discussed.