Borna disease virus (BDV) causes neurological disease in a wide range of animal species, providing an important model for studies of persistent viral infection of the central nervous system. In addition, the detection of antibodies that react with BDV antigen in serum from psychiatric patients suggests a role for BDV, or related viruses, in human mental disorders, providing further reason for study of this poorly characterized neurotropic virus. We present evidence that BDV has a nonsegmented negative single-strand RNA genome with the property that viral replication and transcription take place in the nuclei of infected cells where infectious BDV ribonucleoproteins are present. Our results support the view that BDV has unique biological features among animal viruses. Furthermore, the finding that BDV ribonucleoproteins are able to infect susceptible cells raises interesting questions regarding the mechanisms by which some neurotropic viruses may spread through the central nervous system of the infected host without requiring the production of mature infectious virus.