Abnormalities of glial and mesenchymal elements were studied in six autopsy specimens of alobar holoprosencephaly by histopathological and immunohistochemical methods. Marginal glioneural heterotopia was observed in all of the specimens. It was most prominent in the prosencephalic base around the optic chiasm, and extensive in five specimens. The floor of the third ventricle was intermingled with excessive mesenchymal elements, and in three specimens the ventricular cavity was plugged by heterotopic nodules. In the cerebellum, dysplastic nodules were observed in two specimens. In the telencephalon, the marginal and subventricular layers were thickened. The pathological significance of these findings is discussed with their possible relationship to the pathomechanism of holoprosencephaly.