The hippocampal CA2 subfield was initially identified by Lorente de Nó as an anatomically distinct region based on its cytoarchitectural features. While there is an enormous body of literature on other hippocampal subfields (CA1 and CA3), relatively little is known about the physiological and developmental properties of CA2. Here we report identification of the CA2 region in the mouse by immunostaining with a Purkinje cell protein 4 (PCP4) antibody, which effectively delineates CA3/CA2 and CA2/CA1 borders and agrees well with previous cytoarchitectural definitions of CA2. The PCP4-immunostaining delineated CA2 neurons have distinguishable differences in cell morphology, physiology and synaptic circuit connections as compared to distal CA3 and proximal CA1 regions. The average somatic sizes of excitatory cells differ across CA1-3, with the smallest to largest somatic size being CA1