A canine virus derived from a diseased dog has been plaque-purified and characterized in detail. Analysis of infected cells demonstrated that virus antigen accumulated in the nucleus at 12 to 24 h post-infection and the cytopathology at the ultrastructural level was diagnostic of a parvovirus infection. The purified virus particles were 23 to 26 nm in diam. and banded at a density 1.44 g/ml in CsCl. Detailed biochemical analysis revealed a single-stranded DNA genome and three structural proteins of mol. wt. 82,300, 67,300 and 63,500. All of the data presented are consistent with the classification of this virus as a parvovirus.