The morphology of the virions and nucleocapsids of paramyxovirus 107 (PMV 107) and the replication of the virus were investigated by EM. The virions and nucleocapsids exhibited the same structural properties as other paramyxoviruses. Nucleocapsids were found in the nucleus and cytoplasm of infected bovine embryonic lung (BEL) cell cultures. A similar situation was described for the morbilliviruses measles, SSPE [subacute sclerosing panencephalitis], distemper and rinderpest. Alignment of nucleocapsids beneath the plasma membrane and budding of PMV 107 in the productive BEL cell infections were also similar to the morbillivirus-infected cells. In a line of [African green] monkey [kidney] cells (CV1) persistently infected with PMV 107 only cytoplasmic nucleocapsids could be demonstrated. On the basis of its morphology and morphogenesis it is suggested that PMV 107 should be classified as a paramyxovirus. Since nucleocapsids could also be found in the nucleus of infected BEL cells the morphogenesis of PMV 107 closely resembles that of viruses of the morbillivirus group.