The epithelium of the anal tonsil of the laboratory shrew was studied by scanning and transmission electron microscopy, with particular attention focused on the structure of the epithelium lining the anal tonsillar crypt. The tonsillar crypt surface is lined by two kinds of epithelia: squamous epithelium, which is located mainly at the neck of the crypt and includes keratohyalin granules in the superficial layer, and reticular epithelium, which is invaded by many immigrating cells and has several micropores immigrating cells to pass through. In addition, basal granulated cells are present in the basal layer. These results suggest that the reticular epithelium of the anal tonsil belongs to the welldeveloped gut-associated lymphoid tissue (GALT) in the alimentary canal. It represents a specialized and important compartment in immunological function, similarly to the palatine tonsils of other mammals, and has as yet unknown roles in digestion.