Glial cells constitute nearly 50% of the cells in the human brain(1). Astrocytes, which make up the largest glial population, are crucial to the regulation of synaptic connectivity during postnatal development(2). Because defects in astrocyte generation are associated with severe neurological disorders such as brain tumours(3), it is important to understand how astrocytes are produced. Astrocytes reportedly arise from two sources(4-6): radial glia in the ventricular zone and progenitors in the subventricular zone, with the contribution from each region shifting with time. During the first three weeks of postnatal development, the glial cell population, which contains predominantly astrocytes, expands 6-8-fold in the rodent brain(7). Little is known about the mechanisms underlying this expansion. Here we show that a major source of glia in the postnatal cortex in mice is the local proliferation of differentiated astrocytes. Unlike glial progenitors in the subventricular zone, differentiated astrocytes undergo symmetric division, and their progeny integrate functionally into the existing glial network as mature astrocytes that form endfeet with blood vessels, couple electrically to neighbouring astrocytes, and take up glutamate after neuronal activity.