Seasonal changes of incidence of rice viruses (RRSV, RGSV, RGDV, and RTV) and their insect vectors (Nilaparvata lugens, Nephotettix virescens, N. nigropictus, and Recilia dorsalis) in the field of central Thailand were observed from July, 1979 to December, 1980. In case of RRSV which caused severe damage to the rainy season rice cultivation in some areas of the Central Plain in 1980, the increased incidence of RRSV was closely correlated with a higher percentage of viruliferous insect vectors. From the results obtained, it was deemed possible to forecast the incidence of RRSV on a short term basis by continuous observation of the population and testing the viruliferous percentage of the insect vector.