SUMMARYAutumn‐sown crops of broad beans (Vicia fabaL.) in England often contain plants with some leaves characteristically distorted and with a chlorotic mosaic. From some of these plants true broad‐bean mosaic virus was isolated in 1959 and 1960 but not in 1965 and 1966. From other plants a similar but distinct virus, which caused staining of the seeds and we call broad‐bean stain virus, was isolated in 1960, 1965 and 1966. The two viruses were readily distinguished in serological tests, and in some test plants. Both were seed‐borne, and spread in crops, but were not transmitted by several animal species tested as vectors.Both viruses have isometric particles about 25 mμ in diameter. Some of these particles contain about 35% ribonucleic acid, some about 26% and some of those of broad‐bean stain virus contain none; these three types of particles had sedimentation coefficients of about 120–130S, 100Sand 60Srespectively. The ribonucleic acid of each virus had molar base content of G 23%, A 26%, C 18% and U 32%.These two viruses are members of the cowpea mosaic group of plant viruses; broad‐bean strain virus was serologically related to cowpea mosaic, F I, red‐clover mottle, and squash mosaic viruses. The particles of all these viruses and of true broad‐bean mosaic virus were similar in appearance, sedimentation behaviour, and nucleic acid content and composition. The nucleic acid of red‐clover mottle virus had a molar base content of G 20%,