Morphologic examination was made on 110 brain tumors induced in adult mice by intracerebral inoculation with chicken sarcoma cells producing Schmidt-Ruppin strain of Rous sarcoma virus. The typical gliomas that satisfied the criteria of human ordinary glioma were, in general, induced after a long latency. Histologically, they belonged to glioblastoma, astrocytoma, and their variants. Frequency was 26% of the total tumors and was about 50% of the tumors that developed more than 90 days after inoculation. The tumors composed predominantly of huge round cells were most common throughout the observation period of 400 days and were tentatively designated “huge round-celled tumors.” Frequency was about 70% of the total tumors and about 50% of the tumors developing after 90 days. The glial origin of the huge round-celled tumor was su'ggested from its light and electron microscopical characteristics. Either coexisting with these 2 characteristic tumors or forming independent or discrete tumors, large elongated, neurinoma-like (or spongioblastic), and small spindle cells were also noted. Emphasis was given to the successful induction of the typical glioma in adult mice as well as to the frequent development of a cherccterlstlc huge round-celled tumor that was composed predominantly of huge round cells and resembled human gian.tcelled glioblastoma. Biochemical and biologic evidence for the mouse, not chicken, origin of each brain tumor was also discussed.