The ultrastructure of the cervical cord is described in a new canine mutant with severe hypomyelination of the C.N.S. Axons were either non-myelinated or surrounded by a myelin sheath that was markedly reduced in both its thickness and length of internode. Myelinated and non-myelinated zones were present on a single axon. There was no paucity of oligodendrocytes but many of those present contained empty or granular vacuoles within the cytoplasm. Features suggesting immaturity of myelination were commonly found at paranodes and along the internode. Abnormal inter-relationships of oligodendrocytes and astrocytes were present at many paranodes. These observations suggest an intrinsic defect of oligodendrocyte metabolism such that they are incapable of normal extension of their plasma membranes, while the cytoplasmic vacuoles may represent breakdown of defective lipids.