Efficient assembly of enveloped viruses at the plasma membranes of virus-infected cells requires coordination between cytosolic viral components and viral integral membrane glycoproteins. As viral glycoprotein cytoplasmic domains may play a role in this coordination, we have investigated the importance of the hemagglutinin-neuraminidase (HN) protein cytoplasmic domain in the assembly of the nonsegmented negative-strand RNA paramyxovirus simian virus 5 (SV5). By using reverse genetics, recombinant viruses which contain HN with truncated cytoplasmic tails were generated. These viruses were shown to be replication impaired, as judged by small plaque size, reduced replication rate, and low maximum titers when compared to those features of wild-type (wt) SV5. Release of progeny virus particles from cells infected with HN cytoplasmic-tail-truncated viruses was inefficient compared to that of wt virus, but syncytium formation was enhanced. Furthermore, accumulation of viral proteins at presumptive budding sites on the plasma membranes of infected cells was prevented by HN cytoplasmic tail truncations. We interpret these data to indicate that formation of budding complexes, from which efficient release of SV5 particles can occur, depends on the presence of an HN cytoplasmic tail.