Ultrastructural study of the peripheral nervous system of quaking mice has revealed several unusual features in the pattern of myelination in addition to hypomyelination. These are 1) the presence of 'atypical' Schmidt-Lanterman incisures, 2) irregularities of the nodal and internodal termination of Schwann cell cytoplasm and 3) the presence of non-myelinated segments without associated Schwann cell processes, but covered by basal lamina. In view of the observation of similar features during the development of the P.N.S. and also during P.N.S. remyelination, we suggest that these features are the results of modified myelination due to a defect in the control mechanisms necessary for normal myelination.