INTRODUCTION. In 1970 the Vertebrate Virus Subcommittee of the International Committee on Nomenclature of Viruses proposed the name togavirus (from the latin toga — a cloak) to cover arboviruses having taxonomic characters like those of the serological groups A and B (Andrewes, 1970). The name ‘arbovirus’ should be used in a purely biological sense, i.e. for viruses having a biological cycle in both arthropods and vertebrates; it is endowed with ecological but not with structural significance. During the last few years, a considerable amount of information concerning the structure and multiplication of these agents has been accumulated, confirming that alpha- and flavoviruses, as they are named now (Wildy, 1971), are sufficiently different from other virus groups to justify this classification. The arthropod-borne members of the togavirus family are relatively small spherical RNA-viruses which are sensitive to treatment with organic solvents or mild detergents. The virion is constructed of a spherical core which is wrapped into an envelope carrying projections on its surface.