During thymic development, T cell progenitors undergo positive selection based on the ability of their T cell Ag receptors (TCR) to bind MHC ligands on thymic epithelial cells. Positive selection determines T cell fate, in that thymocytes whose TCR bind MHC class I (MHC-I) develop as CD8-lineage T cells, whereas those that bind MHC class II (MHC-II) develop as CD4 T cells. Positive selection also induces migration from the cortex to the medulla driven by the chemokine receptor CCR7. In this study, we show that CCR7 is up-regulated in a larger proportion of CD4+ CD8+ thymocytes undergoing positive selection on MHC-I compared with MHC-II. Mice bearing a mutation of Th-POK, a key CD4/CD8-lineage regulator, display increased expression of CCR7 among MHC-II-specific CD4+ CD8+ thymocytes. In addition, overexpression of CCR7 results in increased development of CD8 T cells bearing MHC-II-specific TCR. These findings suggest that the timing of CCR7 expression relative to coreceptor down-regulation is regulated by lineage commitment signals.