Adult neurogenesis occurs in the subgranular zone and innermost part of the dentate granule cell layer. To examine how neural precursor cells proliferate, migrate, and extend their neurites, we performed BrdU- and improved retrovirus-green fluorescence protein (GFP)-labeling analyses. Soon after labeling the majority of BrdU+ cells and GFP+ cells expressed Ki67, a cell cycle marker, and formed clusters together with PSA+ neuroblasts. Most of the Ki67+ proliferating cells expressed Hu, an immature and mature neuronal marker, and the subpopulation expressed both Hu+ and GFAP+. In the clusters, Ki67+ and PSA+ cells strongly expressed beta-catenin and N-cadherin, but PSA+ cells outside the clusters did not. Therefore, it was mainly Hu+ neuronal precursor cells that proliferated within clusters in which the cluster cells are closely associated via cell adhesion molecules, such as N-cadherin/beta-cateninIn and PSA. The newly generated cells appeared to stay in the clusters for a few days and then disperse around the clusters. The findings of this in vivo analysis and in vitro time-lapse imaging of early postnatal hippocampal slices support the notion that most postmitotic neuroblasts migrate tangentially from clusters, extending tangentially oriented processes, one of which often retains close contact with the clusters, and finally extend radial processes, or prospective apical dendrites. These results suggest that the clustering cells and tangentially migrating cells have a systematic cellular arrangement and intercellular interaction.