Functions of the cerebral cortex in the central nervous system (CNS) are directly related to its cellular composition, the basics of which are established during early development. Understanding the numerical distribution of neurons and other cortical cells is crucial because during pathophysiological processes of neurodegeneration the number of these cells decreases progressively. In addition, cellular processes that are identical or very similar to developmental processes are activated during tissue regeneration. A unique model for the study of neuroregeneration in mammals is the opossum, which shows the ability to regenerate the CNS through embryonic development until the second postnatal week. The postnatal development of the opossum cortex has not been fully investigated. Data on the total number of neurons and “non-neuronal” cells for the first two postnatal weeks (<P18) have not yet been published, and for the ages older than P18, the only available data include the total number of all cells and neurons. Using the isotropic fractionator (IFR) method and fluorescence microscopy, we quantified cortical cells in opossums of postnatal days (P) 6, 18, 34, and 35. We showed that the absolute number of all cortical cells, including the number of neurons, increased through the postnatal period and that "nonneuronal" cells increase dramatically from P18 to P34, which indicated the end of neurogenesis and the beginning of gliogenesis. We used markers NeuN, SOX2 and Ki-67 to identify and quantify differentiated neurons as well as progenitor and proliferative cells of the opossum cortex in the first postnatal week (P6). Quantification of expression markers indicated the possibility of an immature neuron cells population in the P6 opossum cortex whose progenitor cell characteristics are subsequently silenced.