A virus transmitted by aphid, Aphis glycines, in a persistent manner was detected from dwarfed soybean plants with leaf rolling or rugose symptoms that were collected in Phitsanulok, northern Thailand in 1979. The virus was not transmitted by sap inoculation. Partially purified preparations from infected soybean plants contained spherical particles 25-30 nm in diameter, which reacted positively with antiserum to Indonesian soybean dwarf virus in agar gel double diffusion tests. Ultrathin sections of infected soybean leaves contained aggregations of virus-like particles in phloem cells similar to those of the Indonesian soybean dwarf virus. Based on these results, the virus was identified as Indonesian soybean dwarf virus.