A recombinant baculovirus expressing the nucleocapsid gene (NP) of Newcastle disease virus (NDV), a member of the genus Rubulavirus, has been generated and shown to express the native protein to high levels in insect cells. In contrast to the NP protein of the rubulavirus human parainfluenza virus 2, the NDV protein has been demonstrated by electron microscopy and caesium chloride gradient analysis to be capable of self-assembly in vivo to form nucleocapsid-like structures in the absence of other NDV proteins. These structures, which contained RNA that was resistant to micrococcal nuclease digestion, were also observed when the protein was expressed in E. coli, a phenomenon which was not inhibited by the presence of a 40 amino acid fusion region at the amino terminus of the protein. Further, the formation of these structures was inhibited by the co-expression of the phosphoprotein (P). Therefore, we conclude that the P protein acts as a chaperone, preventing uncontrolled encapsidation of non-viral RNA by NP protein.Newcastle disease virus (NDV) is the aetiological agent for the devastating disease of poultry more commonly known as fowl pest. A member of the family Paramyxoviridae, NDV has recently been reclassified into the genus Rubulavirus (Murphy et al., 1995). Other members of this genus include mumps virus and simian virus 5 (SV5) in addition to the human parainfluenza viruses 2, 4a and 4b (hPIV2, 4a and 4b). The genome of NDV, which comprises a single strand of negative-sense RNA, encodes directly six structural genes in the order 3h-NP-P-M-