The growth of Semliki Forest virus was studied with the electron microscope during a single cycle of viral replication in chick embryo cells. The spherical virus particle consists of a nucleoid closely wrapped in a membrane covered by projections. After a latent period of 2–3 hours, infectious virus is produced at a nearly constant rate of approximately 200 PFU/cell per hour until the ninth hour. During the period of rapid production of virus, individual free virus nucleoids are found scattered in the cytoplasmic matrix. The nucleoids appear to migrate singly to the plasma membrane, where complete virus particles are formed by a budding process. An envelope, consisting of a portion of the membrane covered with projections, encloses a nucleoid as it is extruded into the extracellular space. Few complete virus particles are found within cytoplasmic vacuoles.

After 9 hours, the rate of virus production decreases, and few developing virus particles are seen. Cells begin to show marked cytopathic changes, including vacuolization and condensation of the cytoplasm. In addition to being scattered singly in the cytoplasmic matrix, nucleoids aggregate around vacuoles and in paracrystalline arrays. Such accumulations of nucleoids do not appear to have a functional role in the production of Semliki Forest virus particles.