AbstractPDZ‐domain containing proteins of the MAGUK (membrane‐associated guanylate kinase) family target, anchor, and cluster receptors and channels to subcellular sites. Among the MAGUK proteins, the members of the PSD‐95 family (MAGUKs: PSD‐95, PSD‐93, SAP‐97, and SAP‐102) target and anchor glutamate receptors to the synaptic terminals. Associations of glutamate receptors with MAGUKs have been described in the brain but not in the cochlea. In this study, RT‐PCR, immunofluorescence microscopy, and immunoelectron microscopy were used to investigate the presence and distribution of MAGUK proteins in the organ of Corti. The presence of the mRNA for PSD‐95, PSD‐93, SAP‐97, and SAP‐102 in the organ of Corti was confirmed by RT‐PCR. Immunocytochemistry using a “pan‐MAGUK” antibody, which recognizes all four MAGUK proteins, and selective antibodies against these proteins revealed that all four MAGUKs are present within the base of inner hair cells while all except SAP‐97 are found within the base of the outer hair cells. In addition, PSD‐93 and PSD‐95 are found in postsynaptic afferent terminals on inner hair cells, while postsynaptic afferent terminals on outer hair cells have PSD‐93. Synapse 40:258–268, 2001. © 2001 Wiley‐Liss, Inc.