It has been widely accepted that doublecortin (DCX) may represent a neuronal fate marker transiently expressed by immature neurons during development of the central and peripheral nervous tissue and in neurogenic areas of the adult brain. Previous work described the presence of DCX in the developing dorsal root ganglia (DRG), structures of the peripheral nervous system originating from the neural crest, but no information is available on its expression in adulthood. To this purpose, we have performed an immunohistochemical and biochemical analysis for DCX expression in DRG from adult male mice and rats. To our surprise, we demonstrated that the majority of DRG neurons do express DCX, both in somata and in fibers. DCX+ cells have been characterized morphologically and phenotypically with well-established markers of DRG neuronal subpopulations. A large number of DCX+ cells belong to the small and medium-sized nociceptive neurons. Additionally, DCX immunoreactivity is present in the spinal cord dorsal horns, the projection area of DRG neurons. The novel and unexpected localization for DCX protein opens up new, interesting vistas on the functional role of this protein in mature neurons and in particular in sensory neurons. J. Comp. Neurol. 511:318–328, 2008. © 2008 Wiley-Liss, Inc.