Severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is responsible for SARS infection. Nucleocapsid (N) protein of SARS-CoV encapsidates the viral RNA and plays an important role in virus particle assembly and release. In this study, the N protein of SARS-CoV was found to associate with B23, a phosphoprotein in nucleolus, in vitro and in vivo. Mapping studies localized the critical N sequences for this interaction to amino acid residues 175-210, which included a serine/arginine (SR)-rich domain. In vitro phosphorylation assay showed that the N protein inhibited the B23 phosphorylation at Thr199.