A filamentous virus isolated from Iris japonica with mosaic symptoms was pathogenic to I. japonica and I. bulleyana but not to 12 virus indicator species. Sequencing of the 3'-terminus of the genome showed that it was a potyvirus, most closely related to sunflower mosaic virus (63.2% identical), and phylogenetic analysis also showed a more distant grouping with tobacco etch and Colombian datura viruses. The virus was detected by RT-PCR in symptomatic I. japonica plants from several regions of China. The virus appears to be an isolate of a new species in the genus Potyvirus, tentatively named butterfly flower mosaic virus.