Vertebrate myelin is enriched in the lipid galactocerebroside (GalC) and its sulfated derivated sulfatide. To understand the in vivo function of these lipids, we analyzed myelination in mice that contain a null mutation in the gene encoding UDP-galactose:ceramide galactosyltransferase, the enzyme responsible for catalyzing the final step in GalC synthesis. Galactolipid-deficient myelin is regionally unstable and progressively degenerates. At postnatal day 30, demyelination is restricted to the midbrain and hindbrain, but by postnatal day 90, it spreads throughout the central nervous system. Activated microglial cells and reactive astrocytes appear with the loss of myelin in older animals. Nonetheless, major myelin protein gene mRNA levels are normal throughout the life of these animals, suggesting that widespread oligodendrocyte death is not the primary cause of demyelination. The developmental switch in myelin-associated glycoprotein isoform expression, however, does not occur normally in these mice, suggesting an alteration in oligodendrocyte maturation. Taken together, these findings indicate that GalC and sulfatide are required for the long-term maintenance of myelin and that their absence may have subtle effects on the development of oligodendrocytes.