The spinal cord, optic nerves, and cerebellum of the mouse mutant Shiverer were examined by electron microscopy of thin sections. Although central nervous system myelin is grossly deficient in amount, none of its basic structural elements are missing. Regions of compact myelin can be found composed of several layers of alternating major dense lines and intermediate lines repeating with normal periodicity. The "radial component" consisting of periodic thickenings of the intermediate line aligned through several lamellae was also identified. Axoglial junctions characteristic of the type found in paranodal regions are present in greater than normal numbers but occur in aberrant locations. Myelin sheaths have marked reduced numbers of lamellae, which often contain cytoplasm, terminate in cytoplasmic "loops" within and around myelin sheaths, and do not completely encircle axons. In addition, membranous debris appears within neuronal and glial profiles, suggesting some degree of myelin breakdown. Thus, the protein lacks in this mutant appear not to be associated with discrete deficiencies of specific structural components but rather with a variety of quantitative changes and irregularity of form.