Tenascin-X (TNX) is an extracellular matrix protein that is highly expressed in the peripheral nervous system as well as muscular tissues, especially the heart and skeletal muscle. However, the expression manner and the physiological role of TNX in the peripheral nervous system have not been fully investigated. In this study, we elucidated its distribution in adult mouse sciatic nerves by immunohistochemical staining. TNX was found to be localized in the perineurium and the endoneurium of sciatic nerve fibers. To examine the physiological role of TNX, we investigated sciatic nerves of TNX-deficient mice that are viable and fertile and have no obvious deficits in general performance. The thickness of myelin sheaths and the size of the individual axons in these mice appeared normal. The ultrastructure of the sciatic nerves of TNX-deficient mice were similar to those of wild-type mice. Thus, the lack of a discernible phenotype in the sciatic nerves of TNX-deficient mice suggests that TNX has either a redundant or a very subtle function in the macromolecular organization in the peripheral nerve.