Cells within the subventricular zone (SVZ) express basic Fgf <i>(Fgf2)</i> and Fgf receptor proteins. We show that the absence of <i>Fgf2</i> gene products reduces by 50% the dividing progenitor population of the anterior SVZ (SVZa) without changing their cell cycle time. Every 2–3 cell cycles of the SVZa progenitor cell population, 30,000 newly generated neurons capable of long-term survival are added to the glomerular layer of the olfactory bulb. <i>Fgf2</i> knockout mice have smaller olfactory bulbs due to decreased output of these newly generated cells into the bulbs. A population of slow-dividing neural stem cells (NSCs) residing in the SVZa is identified by its slow cell cycle kinetics (cell cycle approx. 20 days); these cells, called ‘S’ cells, are negative for glial fibrillary acidic protein and occasionally express brain-lipid-binding protein, a molecular marker of radial glia. The number of these dividing NSCs is reduced from about 13,000 in wild-type to 8,500 cells in <i>Fgf2</i> knockout mice. Thus, <i>FGF2</i> regulates the number of proliferative cells and olfactory bulb neurogenesis by maintaining a slow-dividing stem cell pool within the SVZa.