Electronmicroscopic studies in a case of myomedulloblastoma support the concept that the medulloblastoma and the myoblastoma component of the tumor derive drom different cell lines. The myoblastoma component shows bundles of myogenic elements, whereby quite undifferentiated cells lacking myofilaments are tightly attached to differentiated elements having well organized myofilaments. These bundles are encompassed by a common basement membrane, and their cells are joined by junctions, helping the identification of the undifferentiated elements. No comparable structures were seen in the medulloblastoma component. The fine structure of the myoblastoma is reminiscent of aspects of normal fetal development of skeletal muscle.