The ultrastructural changes of typical lesions in canine globoid cell leukodystrophy (GLD) have been studied. The globoid cells were located in the cerebral parenchyma as well as in the perivascular Virchow-Robin space. Features suggestive of a passage of the globoid cells from the cerebral parenchyma to the Virchow-Robin space were also observed through the interruptions in the basal lamina. The globoid cells had numerous thin pseudopods and contained various cytoplasmic inclusions which have been described previously. Detailed studies of these inclusions suggest that they represented aggregates of filamentous or linear sub-unit structures. Typical oligodendroglial cells were found on only a few occasions. Both globoid cells and oligodendroglia contained myelin debris, dense bodies and honey-comb like inclusions composed of numerous small myelin figures. In a few instances, crystalline polygonal inclusions identical to those found in the globoid cells, were found in the cytoplasm of the cells which were, with reasonable certainty, identifiable as oligodendroglia. In less affected areas where myelin was still present, degenerating oligodendroglia, with or without recognizable inclusions, were frequently encountered. Astrocytes and endothelial cells contained concentric lamellar inclusions and dense bodies but did not contain the tubular inclusions as seen in globoid cells. The possible significance of the ultrastructural features in regard to the pathogenesis of the GLD have been discussed.