Cucumber mosaic virus designated as CMV-M was isolated from winged bean showing ringspot on leaves in Thailand and from asparagus bean showing mosaic in Indonesia. The host ranges of CMV-M were wide, but the symptoms on most plants were mild or latent. CMV-M was transmitted by aphids in a non-persistent manner. Electron micrographs of partially purified preparations of CMV-M revealed the presence of isometric particles about 30 nm in diameter. CMV-M was closely related serologically to the yellow strain of CMV (CMV-Y). Prior infection of tobacco with CMV-M gave incomplete protection against systemic infection of CMV-Y or a strain of CMV isolated from tomato.