SummaryA hitherto undescribed virus, termed rottboellia yellow mottle virus (RoYMV), affectingRottboellia cochinchinensis(syn.R. exaltata, itch grass) at Ibadan, Nigeria, was investigated.RoYMV virions are isometric,c. 29 nm in diameter, and sediment homogeneously at 114s. In isopycnic CsCl gradients, RoYMV virions band sharply at a buoyant density of 1.379 g cm‐3, but in Cs2SO4gradients, virions band at two zones, at the densities of 1.300 g cm‐3and 1.325 g cm‐3. Treatment with EDTA at pH 8.0 reduced the sedimentation value of RoYMV to c. 87s and rendered it susceptible to proteinase‐K, SDS and NaCl. The apparent molecular weight of RoYMV coat protein wasc. 27 000. Virions encapsidate a single‐stranded RNA of mol. wt 1.4 × 106Da. BesidesR. cochinchinensis, RoYMV was mechanically transmissible only to maize (Zea mays). No reaction occurred when RoYMV was tested against antisera to 44 isometric plant viruses (belonging to 12 groups), including several that affect Gramineae.RoYMV exhibits striking similarities with other viruses belonging to the sobemovirus group, and it is tentatively designated as a new member of the sobemov