The lipid composition of CNS and PNS myelin was studied in rat, Xenopus, trout and Torpedo. The main difference lay in the proportion of cerebrosides, which decreased in the sequence rat greater than Xenopus greater than Torpedo greater than trout. In addition Torpedo CNS and PNS myelins were extremely rich in sulfatides. In some respects, Torpedo appeared closer to tetrapods than trout. Otherwise the proportion of the different lipid classes did not reveal any clear evolutionary trends. The presence of hydroxylated galactolipids in CNS myelin was investigated in several additional species. Considerable amounts were found in Torpedo, Polypterus, Protopterus, lizard, and chicken, with the highest values in rat and anurans. Only very small amounts of hydroxylated cerebrosides were detected in trout and in axolotl, while newt had none. This parameter appears therefore of doubtful usefulness for phylogenetic studies. In contrast to myelin proteins, myelin lipids are of limited value for establishing phylogenetic relationships among vertebrates.