Respiratory syncytial virus (RSV), a nonsegmented, negative-sense RNA-containing virus, is a common cause of lower respiratory tract disease. Expression of RSV nucleocapsid protein (N) in insect cells using the baculovirus expression system leads to the formation of N-RNA complexes that are morphologically indistinguishable from viral nucleocapsids. When imaged in an electron microscope, three distinct types of structures were observed: tightly wound short-pitch helices, highly extended helices, and rings. Negative stain images of N-RNA rings were used to calculate a three-dimensional reconstruction at 24 A resolution, revealing features similar to those observed in nucleocapsids from other viruses of the order Mononegavirales. The reconstructed N-RNA rings comprise 10 N monomers and have an external radius of 83 A and an internal radius of 40 A. Comparison of this structure with crystallographic data from rabies virus and vesicular stomatitis virus N-RNA rings reveals striking morphological similarities.