Chemokines appear to have an important role in the seeding of lymphoid progenitors in the thymus, the regulation of the coordinated movements of the maturing T cells within this organ, and the egress of the resulting naive T cells to secondary lymphoid organs. CCR9, the specific receptor for the β-chemokine TECK/CCL25, is selectively expressed in thymus, lymph node, and spleen. Using a specific anti-CCR9 polyclonal antibody, K629, and a semiquantitative reverse transcriptase–polymerase chain reaction procedure, a detailed study of CCR9 expression in the thymus and secondary lymphoid organs was performed. The results show that CD4⁺CD8⁺ double-positive thymocytes have the highest CCR9 expression in thymus. Single-positive CD8⁺ thymocytes continue to express this receptor after abandoning the thymus as mature naive T cells, as suggested by the existence of a CD8⁺CD69^lowCD62L^highCCR9⁺ cell subset. Consistent with this, CD8⁺lymphocytes from lymph nodes, spleen, and Peyer patches express a functional CCR9, as its expression correlates with migration in response to CCL25. Conversely, CD4⁺ thymocytes lose CCR9 before abandoning the thymus, and CD4⁺ T cells from secondary lymphoid organs also lack CCR9 expression. Analysis of CCR9 expression in thymocytes from mice of different ages showed that CCR9 levels are affected by age, as this receptor is more abundant, and its response to CCL25 is more potent in newborn animals. Collectively, these results suggest that CCR9 has a role in thymocyte development throughout murine life, with clear differences between the CD4⁺ and CD8⁺ lineages.