Microglia are increasingly implicated as a source of non‐neural regulation of postnatal neurogenesis and neuronal development. To evaluate better the contributions of microglia to neural stem cells (NSCs) of the subventricular neuraxis, we employed an adherent culture system that models the continuing proliferation and differentiation of the dissociated neuropoietic subventricular tissues. In this model, neuropoietic cells retain the ability to self‐renew and form multipotent neurospheres, but progressively lose the ability to generate committed neuroblasts with continued culture. Neurogenesis in highly expanded NSCs can be rescued by coculture with microglial cells or microglia‐conditioned medium, indicating that microglia provide secreted factor(s) essential for neurogenesis, but not NSC maintenance, self‐renewal, or propagation. Our findings suggest an instructive role for microglial cells in contributing to postnatal neurogenesis in the largest neurogenic niche of the mammalian brain. � 2006 Wiley‐Liss, Inc.