Telencephalin is an intercellular adhesion molecule (ICAM) restricted to the telencephalon. This study demonstrates that immunolabeled telencephalin is targeted to the somatodendritic domain of cultured hippocampal pyramidal neurons beginning with the first stages of dendritic differentiation. In contrast, it is entirely excluded from all gamma-aminobutyric acid (GABA)ergic inhibitory interneurons at all stages of development. Prior to the stage at which nearly all pyramidal neurons express telencephalin, labeled neurons possess more extensive dendritic arbors than unlabeled pyramidal neurons. More synaptic boutons form with the larger, more elaborate telencephalin-expressing dendritic trees, but bouton number per unit length is similar between neurons with and without telencephalin. These findings suggest that telencephalin identifies pyramidal neurons, it may identify plasma membrane as dendritic, and it may be generally adhesive or stabilize dendritic membranes, but it is probably not specifically synaptic. Such characteristics would be expected to be important for the formation of cell type-specific dendritic arbors.