The development of the neocortex of the marsupial Monodelphis domestica has been studied from birth until adulthood. Monodelphis is born after a gestational period of 14 days, a time when the neocortex is still at a two-layered "embryonic" stage of development, that is equivalent to a 13-14 day rat embryo or 6 week human embryo. The cortical plate does not begin to appear until 3 to 5 days postnatal. Thus the whole of neocortical development is a postnatal phenomenon in this species, as has been previously described in other marsupials. The general pattern of development of the characteristic layers of the immature neocortex and the subsequent development of a six-layered adult neocortex is similar to that found in eutherian species. However there are some differences. The depth of the immature cortical plate when compared to the thickness of the neocortical wall is less than in eutherians and the subplate zone is much deeper in Monodelphis; this transient subplate zone consists of widely spaced rows of cells that are aligned parallel to the cortical surface. Unlike eutherians there appears to be no secondary proliferative zone in the subventricular zone of the dorso-lateral neocortical wall. Maturation of the neocortex is apparent by 45 days postnatal and by 60 days (around the time of weaning) the characteristic six-layered adult neocortex is clearly present. The neuronal marker PGP 9.5 was used to define neuronal populations in the adult brain. The density of neurons in Monodelphis appears to be considerably less than in eutherians such as the rat. The suitability of postnatal Monodelphis for studies of neocortical development is discussed.