Although many viruses replicate in the nucleus, little is known about the processes involved in the nuclear import of viral genomes. We show here that in vitro generated core particles of human hepatitis B virus bind to nuclear pore complexes (NPCs) in digitonin-permeabilized mammalian cells. This only occurred if the cores contained phosphorylated core proteins. Binding was inhibited by wheat germ agglutinin, by antinuclear pore complex antibodies, and by peptides corresponding either to classical nuclear localization signals (NLS) or to COOH-terminal sequences of the core protein. Binding was dependent on the nuclear transport factors importins (karyopherins) alpha and beta. The results suggested that phosphorylation induces exposure of NLS in the COOH-terminal portion of the core protein that allows core binding to the NPCs by the importin- (karyopherin-) mediated pathway. Thus, phosphorylation of the core protein emerged as an important step in the viral replication cycle necessary for transport of the viral genome to the nucleus.