Ultrathin sections of brain tissue from mice infected with three Bunyamwera group arboviruses, Bunyamwera, Tensaw, and Maguari, were examined by electron microscopy. In all three cases, virus was observed budding from membranes into intracytoplasmic cisternae. Mature virus was found accumulated within these distended cisternae and in extracellular spaces. The three viruses were morphologically indistinguishable; particles were round or oval, 98 nm in diameter, and had a closely adherent irregular envelope.