The marginal zone (MZ) is the most superficial layer of the developing telencephalon which will become cell sparse adult cortical layer I. In this study the MZ in human and cynomologus monkey was examined on Nissl and immunohistochemically processed postmortem fetal brain tissue. From 7 to 13 postconceptional week (pcw) in human, and from embryonic day (E) 46 to 56 in monkey, large cells that express calbindin and calretinin were observed in the MZ, but the overall cell density was exceptionally low. At 15pcw and E64 cell density increased, and small cells that express calretinin, calbindin, somatostatin or glutamate-decarboxylase (GAD) were observed in the MZ. From 17 to 20pcw in human, and E68 to 83 in monkey a further increase in cell density was observed. The thickness of the MZ doubled and the following sublayers were distinguished: Subpial layer with densely arranged small granular GAD65 and GAD67 expressing cells distributed between numerous large cells that express calretinin or calbindin; Transparent layer with less densely packed small granular cells; Thick layer with densely arranged GAD67 expressing granular cells; Transparent layer containing mainly densely packed fibers oriented parallel to pial surface and reactive for calretinin or calbindin; Thin layer of GAD67 expressing granular and pyriform cells; Thin transparent layer above the cortical plate. The six-layer sublamination observed at midgestation in human and monkeys is the most complex form of organization of the MZ during the whole lifespan, and is so far only described in primate species.