Aprosencephaly/atelencephaly is an extremely rare central nervous system defect of the human fetus, which has been attributed to an encephaloclastic process. We report here the histopathological features of the rudimentary forebrain and retina of a case of aprosencephaly in a male fetus that was examined at 20 weeks of gestation, in which there was no evidence of destruction. The prosencephalon was excessively small and replaced by a solid, cylindrical mass without a hemispheric cleavage or a ventricle. Histologically, the rudimentary forebrain tissue consisted of primitive cells that had accumulated in the anterior portion, and of differentiated neurons and glia that were distributed in a random-manner in the posterior portion. The most rostral portion of the tissue penetrated the sphenoidal bone through a single canal: an extracranial ectopic nest consisted of primitive cells that were located at the posterior portion of the face. There were no eyelid openings, eyeballs, or optic nerves, although a nest of dysplastic retinal rosettes and two nests of melanin-pigmented epithelium were identified. This case seems to represent a primary malformation, and the features imply that the failure occurs after neurulation and before the prosencephalic medium began its growth.