Electron microscopic observation was carried out on epithelial cells of Lieberkuhn's crypts in the natural case of canine parvovirus infection. The replication of virions was revealed in the nuclei of the lining epithelial cells, as demonstrated in feline panleukopenia. In cells seemingly in the early stage of infection, virus particles 18 to 20 nm in diameter were scattered in swollen nuclei. In rather severely infected cells, the nuclei were considerably atrophic with a marked margination of chromatin granules and occupied with fine fibrillar masses and virion clusters. The nuclei of epithelial cells desquamated into the lumen were shown to have compact crystalline arrays of virions. Virions were observed also in the cytoplasm, being either membrane-associated or free in the ground substance.