UDP-galactose:ceramide galactosyltransferase (CGT) is considered to be a key enzyme in the biosynthesis of galactocerebroside (GalC), a frequently used marker for oligodendrocytes. CGT gene expression has been shown to be restricted to the oligodendrocyte lineage in young adult rat brain. We detected expression of the CGT gene in embryonic mouse brain and spinal cord from E17 onward using an in situ hybridization technique. Localization of CGT messages was restricted to two longitudinal columns of cells in the basal half of the ventricular zone around the fourth ventricule at the most caudal level of the medulla oblongata. After birth, they were distributed in caudal-to-rostral and ventral-to-dorsal gradients. This study shows that CGT mRNA is a good marker for localizing oligodendrocytes, and appears just before the GalC-positive stage.