Degradation of purified rubella virus by heat treatment (37, 45, or 56 °C) revealed the following structures. The viral envelope, a modified cellular membrane, bears spherical subunits, 5–6 nm in diameter, hexamers, or pentamers. Two glycoproteins, VP-2 (50 000 daltons) and VP-3 (63 000 daltons), are associated with the envelope. The nucleocapsid is formed by the condensation of the viral ribonucleic acid on a central structure 10 nm in diameter. Only one protein, VP-1 (35 000 daltons) is present in the nucleocapsid. Similarity between rubella virus and Togaviruses is discussed.