Tissue-cultured seedlings of calla lily (Zantedeschia spp.) with mild mosaic symptom were observed in Taiwan. A new potyvirus, Zantedeschia mild mosaic virus (ZaMMV), was identified in the diseased plants based on the result of ELISA, virion morphology and sequence data. In the host range test, ZaMMV only infected Philodendron selloum and Zantedeschia spp. causing systemic mosaic and mild mosaic symptom, respectively. The 3'-terminal region of the virus was amplified by RT-PCR from total RNA using a potyvirus-specific degenerate primer and an oligo-dT primer. Sequence analysis revealed that the highest amino acid sequence identity of the capsid protein (CP) gene between ZaMMV and thirty potyviruses was 55%, and the highest nucleotide identity of 3' untranslated region (3'UTR) was 52%. Therefore, in terms of species demarcating criteria, ZaMMV is a new species of the genus Potyvirus. From the result of phylogenetic analysis, ZaMMV should be a member of the Bean common mosaic virus subgroup. According to the deduced amino acid sequence of ZaMMV, the N terminus of the CP contained 39 glutamine residues before DAG motif. This unique sequence has never been discovered in plant viruses.