Adult neural stem cells are rare, and little is known about their unique characteristics, leaving their in vivo identity enigmatic. We show that Lewis X (LeX), a carbohydrate expressed by embryonic pluripotent stem cells, is made by adult mouse subventricular zone (SVZ) stem cells and shed into their environment. Only 4% of acutely isolated SVZ cells are LeX(+); this subpopulation, purified by FACS, contains the SVZ stem cells. Ependymal cells are LeX(-), and purified ependymal cells do not make neurospheres, resolving the controversial claim that these are stem cells. Thus, LeX expression by adult CNS stem cells aids their in vivo identification, allows their enrichment, and raises new questions about the role of this unusual carbohydrate in stem cell biology.