Twenty two cases of cerebral and so-called cerebellar astrocytomas were heterologously transplanted to guinea pigs. Although the fifteen cerebral astrocytomas were selected according to optimal conditions for transplantation, all failed to grow heterologously. The cerebral astrocytomas represent varieties of fibrillary, protoplasmic and pilocytic types, and all originated in adults. Of the seven randomly selected so-called cerebellar astrocytomas, all of which occurred in children, only one grew when transplanted intracerebrally to guinea pigs. This tumor before transplantation had all the histological characteristics of a so-called cerebellar astrocytoma. The prevailing cells were elongated, spongioblast-like elements; however, other cell types, e.g., astrocytes occurred in varying numbers. The heterotransplant was dominated by elongated, mostly bipolar cells with a very characteristic cytoarchitectural arrangement. The controversy regarding classification of cerebellar astrocytomas versus cerebellar spongioblastomas was discussed. Although the name “spongioblastoma” is not favored, the concept of classifying the so-called cerebellar astrocytoma together with other slow growing midline gliomas as proposed by Zulch has definite merits. Heterotransplantation studies clearly demonstrate that the so-called cerebellar astrocytoma has different biological properties than cerebral astrocytomas.