Oligodendrocytes form an insulating multilamellar structure of compact myelin around axons, thereby allowing rapid propagation of action potentials. Despite the considerable clinical importance of myelination, little is known about the molecular mechanisms that enable oligodendrocytes to generate their specialized membrane wrapping. Here, we used microarray expression profiling of oligodendrocyte-ablated mutant mice to identify new glial molecules that are involved in CNS myelination. This effort resulted in the identification of Ermin, a novel cytoskeletal molecule that is exclusively expressed by oligodendrocytes. Ermin appears at a late stage during myelination, and in the mature nerves, it is localized to the outer cytoplasmic lip of the myelin sheath and the paranodal loops. In cultured oligodendrocytes, Ermin becomes visible in well differentiated MBP-positive cells, where it is concentrated at the tip of F-actin-rich processes (termed "Ermin spikes"). Ectopic expression of Ermin, but not of a mutant protein lacking its actin-binding domain, induced the formation of numerous cell protrusions and a pronounced change in cell morphology. Our results demonstrate that Ermin is a novel marker of myelinating oligodendroglia and suggest that it plays a role in cytoskeletal rearrangements during the late wrapping and/or compaction phases of myelinogenesis.