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This article deales with the description of a newly discovered virus, pea seed-borne mosaic virus (PSbMV). The virus is found to be widespread in pea and broadbean in Wakayama-Prefecture, Japan. Slight chlorosis of plant and leaf curling are the marked characteristic symptoms in peas. Vein chlorosis, mosaic and plant stunting are also the common symptoms. PSbMV is transmitted by plant juice and 4 species of aphids, such as Aphis craccivora, Myzus persicae, Rhopalosiphum padi and Macrosiphum sp., and also through seeds. The rate of seed transmission in pea is found to be about 30% in Oranda, 10% in Sanjunichi-Kinusaya and Futsukoku-Osaya, and over 20 and 10% in New Season and Perfected Wales. PSbMV is inactivated at the temperatures of 55 to 60°C with 10 minutes exposure, at dilutions of 10-3 to 10-4, and in the aging of 4 to 8 days at 20°C. Pea, broadbean, sweetpea, common vetch, hairly vetch and milk vetch are susceptible to the virus systemically. Only local infections are noticed in Chenopodium amaranticolor and Tetragonia expansa. In a few varieties of bean, local lesions are formed very occasionally. Other plants tested, 33 species in 10 families, are all found to be not susceptible to PSbMV. Particles of the virus in dip and partially purified preparations under the electron microscope are found to be flexible filaments 750mμ in length and about 13mμ in diameter. Both positive and negative results of cross protection test were obtained among PSbMV and two different isolates of bean yellow mosaic virus.