CDO is a cell-surface protein of the immunoglobulin/fibronectin type III repeat family that positively regulates myogenic differentiation in vitro. To gain a better understanding of the role of cdo during vertebrate development, we carried out an extensive in situ hybridization study to characterize its expression pattern from postimplantation to late stages of mouse embryogenesis and in rat brain from E13 to adult. Our results show a broad pattern of cdo expression that is spatially and temporally restricted during embryogenesis. In the central nervous system (CNS), cdo expression is detected as early as E7.5 and maintained in the dorsal ventricular zones of the brain and spinal cord, becoming increasingly restricted in the adult. High levels of cdo are detected in developing sensory organs, such as the eye and ear. Outside the CNS, cdo is expressed mainly in neural crest and mesodermal derivatives, including skeletal muscle precursors. Overall, the highest levels of cdo expression are seen from E9.0 to E15.5. The temporal onset and restricted expression of cdo suggest that cdo plays a role in the determination and/or differentiation of a number of cell types during embryogenesis.