A new disorder of central myelination was found in black mutant hamsters showing trembling. The condition was inherited in an autosomal recessive mode. Histologically, a marked myelin deficiency was distributed throughout the central nervous system (CNS), and it was closely related to clinical manifestations observed in the mutants Ultrastructurally, abnormally thin myelin sheaths were observed in all axons with different diameters, and most axons below 1 micron in diameter were unmyelinated in the white matter of the spinal cord. There was no overt evidence of demyelination, but some myelin sheaths showed an aberrant configuration. The numbers of glial cells were not reduced, and there were no striking morphological abnormalities in them. It was suggested that there may be a dysfunction of oligodendrocytes resulting in retarded or defective myelination. This trembling hamsters may be a suitable animal model for the study of normal and abnormal myelinogenesis.