Technical Abstract: Miscanthus is being evaluated as a bioenergy feedstock because of its potentially significant biomass production, perennial habit, and lack of major diseases and pests. It is also a valuable parent in the sugarcane breeding program as source of cold tolerance. Mosaic symptoms were observed on a clone of Miscanthus sinenesis Anderss. maintained at the USDA, ARS, Sugarcane Research Unit in May 2010. The plant tested positive using reverse transcription-polymerase chain reaction (RT-PCR) with the Potyvirus Group PCR Test (Agdia, Inc., Elkhart, IN) using primers sets Poty1F/Poty1R and Poty2F/Poty2R. After sequencing the potyvirus amplicons, a BLAST search in GenBank resulted in the highest identities (81% and 69%) with Sorghum mosaic virus (SrMV) at the nucleic acid level and a 72 and 95% similarity at the amino acid level. Extracts from the Miscanthus clone also tested positive for potyvirus by indirect enzyme-linked immunoasorbent (ELISA) using the ‘universal’ potyvirus monoclonal antibody, PTY1. To our knowledge, this is the first report of a potyvirus on Miscanthus. Mosaic caused by SrMV could become an economically important disease of Miscanthus if grown for bioenergy feedstock on large crop areas. Additionally, a SrMV-infected Miscanthus crop could pose a threat to established crops of susceptible sugarcane and sorghum.