A previously uncharacterized virus with flexuous filamentous particles 660 nm in length was identified in the United States in florists' hydrangea (Hydrangea macrophylla), in which it caused chlorotic mottling, leaf deformation, and discoloration. The virus, tentatively named Hydrangea chlorotic mottle virus (HdCMV), was transmitted readily by mechanical inoculationand by Myzus persicae, but infected only H. macrophylla. The amino acid sequence of a 1.7-kb amplicon comprising the 3? terminus of the HdCMV genome contained one partial and three complete putative open reading frames (ORFs) most similar in size, arrangement, and sequence to the homologous regions of the genomes of known carlaviruses. Based on virion morphology, genome properties, and current criteria for species demarcation, it was concluded that HdCMV represented a new species in the genus Carlavirus. Hydrangea ringspot virus (HdRSV, genus Potexvirus) occurred in mixed infections with HdCMV, but the two viruses could be distinguished readily by serological tests.