The Mongolian gerbil is an easily maintained rodent which has been useful for investigations of the middle and inner ear. The gerbil may be used as an animal model for otitis media and cholesteatoma. The purpose of this study was to describe the ultrastructure of the epithelial and subepithelial lining of the gerbilline middle ear. The middle ear of the gerbil is lined by an epithelial membrane which is similar in cell type to human and guinea pig middle ear mucosa. The major part of the middle ear is lined with simple, non-keratinizing squamous cells with or without microvilli. Ciliated and secretory cells were concentrated around the Eustachian tube orifice; additionally, ciliated cells were seen in two distinct bands extending posteriorly below the cochlea in the hypotympanum and above the cochlea toward the tensor tympani muscle. The subepithelial region consists of fusiform cells (fibroblasts or 'surface osteocytes' and collagen adjacent to bone. These cell types and their distribution further supports the use of the gerbil as a model for human disease.