AbstractGranule cells of the mouse fascia dentata are widely used in studies on neuronal development and plasticity. In contrast to the rat, however, high‐resolution morphometric data on these cells are scarce. Thus, we have analyzed granule cells in the fascia dentata of the adult Thy1‐GFP mouse (C57BL/6 background). In this mouse line, single neurons in the granule cell layer are GFP‐labeled, making them amenable to high‐resolution 3D‐reconstruction. First, calbindin or parvalbumin‐immunofluorescence was used to identify GFP‐positive cells as granule cells. Second, the dorsal‐ventral distribution of GFP‐positive granule cells was studied: In the dorsal part of the fascia dentata 11% and in the ventral part 15% of all granule cells were GFP‐positive. Third, GFP‐positive and GFP‐negative granule cells were compared using intracellular dye‐filling (fixed slice technique) and patch‐clamp recordings; no differences were observed between the cells. Finally, GFP‐positive granule cells (dorsal and ventral fascia dentata) were imaged at high resolution with a confocal microscope, 3D‐reconstructed in their entirety and analyzed for soma size, total dendritic length, number of segments, total number of spines and spine density. Sholl analysis revealed that dendritic complexity of granule cells is maximal 150–200 μm from the soma. Granule cells located in the ventral part of the hippocampus revealed a greater degree of dendritic complexity compared to cells in the dorsal part. Taken together, this study provides morphometric data on granule cells of mice bred on a C57BL/6 background and establishes the Thy1‐GFP mouse as a tool to study granule cell neurobiology. © 2008 Wiley‐Liss, Inc.