Serological and physical properties of isolated particles as well as reactions of barley cultivars after virus inoculation showed that the yellow disease of barley is caused by different viruses. Results summarized in this paper suggest that the European isolates BaYMV-So and BaYMV-NM are closely related to, or identical with, BaYMV-J first reported from Japan, whereas BaYMV-M differs in several properties. Therefore, it seems justified to describe BaYMV-M as a separate virus tentatively designated barley mild mosaic virus.