Morphologic characteristics of most of known oncogenic viruses, some of which were investigated by us, were described in relation to their oncogenicity and their localizations in tumor tissues. Oncogenic viruses were classified into six groups: type A, B, C, D, E and F. Viruses pertaining to type A were presumably not mature virus particles. Some of them could, however, be immature form or incomplete form of oncogenic virus particles. Viruses belonging to type B, C, D, E, and F were represented by mammary tumor virus of mice, leukemia viruses of mice and chickens, polyoma virus, Shope fibroma virus and Lucke renal adenocarcinoma virus, respectively. They were different in size and shape, and in the mode of development. It was presumed that type B and C viruses were members of the Myxovisus group and type D viruses belonged to Adenovirus and the Papova virus group. It was also believed that type E and F viruses had in general the feature of the Pox virus group and Herpes virus group, respectively.