Attempts were made to localize the site of replication of polyoma virus DNA in mouse embryo cells. Although newly synthesized viral DNA was clearly confined within the nucleus, no evidence was found to indicate that replication takes place either on the nuclear membrane, or in the nucleolus, or in association with any other large structure. In all nuclear fractions examined, the mature and replicative forms of the viral DNA were found in the same proportions, but as parts of DNA–protein complexes with unique sedimentation properties.