Rice grassy stunt (RGS) was associated with the presence of virus-like filamentous and small isometric particles. Filamentous particles were 6-8 nm in diameter, 950-1350 nm long, and often circular. The filamentous particles consisted of ribonucleoprotein. Antiserum to the nucleoprotein specifically reacted with extracts of RGS-affected plants and the vector brown planthopper Nilaparvata lugens exposed to RGS-affected plants. Serologically, the nucleoprotein was distantly related to rice stripe virus. The name rice grassy stunt virus (RGSV) is proposed for the filamentous nucleoprotein. RGSV was efficiently detected in RGSV-infected rice leaves and RGSV-exposed planthoppers by ELISA and latex agglutination test. ELISA has been applied to monitor RGSV carriers in migrating planthoppers. A new RGSV strain causing tungro-like symptoms occurred in the Philippines. The strain shows a pathogenicity to rice cultivars with a resistance gene. Similar strains also occurred in India and Thailand.