The nucleocapsid protein (NP) of mumps virus (MuV), a paramyxovirus, was coexpressed with the phosphoprotein (P) in Escherichia coli. The NP and P proteins form a soluble complex containing RNA. Under a transmission electron microscope, the NP-RNA complex appears as a nucleocapsidlike ring that has a diameter of approximately 20 nm with 13 subunits. There is a piece of single-stranded RNA with a length of 78 nucleotides in the NP-RNA ring. Shorter RNA pieces are also visible. The MuV NP protein may provide weaker protection of the RNA than the NP protein of some other negative-strand RNA viruses, reflecting the degree of NP protein association.