During the last 5 years, blackberry plants in Arkansas and North and South Carolina exhibited virus‐like symptoms of vein yellowing and mosaic, followed in some cases by death. Diagnostic tests for known blackberry viruses failed to identify a causal agent. Double‐stranded RNA was extracted from affected plants and cloned. A new member of the Closteroviridae was identified and designated Blackberry yellow vein associated virus (BYVaV). Molecular and immunological assays have been developed for BYVaV, and examination of plants with symptoms revealed a close association of disease symptoms with the presence of BYVaV, although the virus was also found in symptomless plants. Molecular characterization of isolates from plants exhibiting different degrees of disease severity indicated that sequence diversity is probably not the cause of the observed phenotypic differences.