The thymus generates T cells with diverse specificities and functions. To assess the contribution of cytokine receptors to the differentiation of T cell subsets in the thymus, we constructed conditional knockout mice in which IL-7Rα or common cytokine receptor γ chain (γc) genes were deleted in thymocytes just before positive selection. We found that γc expression was required to signal the differentiation of MHC class I (MHC-I)–specific thymocytes into CD8+ cytotoxic lineage T cells and into invariant natural killer T cells but did not signal the differentiation of MHC class II (MHC-II)–specific thymocytes into CD4+ T cells, even into regulatory Foxp3+CD4+ T cells which require γc signals for survival. Importantly, IL-7 and IL-15 were identified as the cytokines responsible for CD8+ cytotoxic T cell lineage specification in vivo. Additionally, we found that small numbers of aberrant CD8+ T cells expressing Runx3d could arise without γc signaling, but these cells were developmentally arrested before expressing cytotoxic lineage genes. Thus, γc-transduced cytokine signals are required for cytotoxic lineage specification in the thymus and for inducing the differentiation of MHC-I–selected thymocytes into functionally mature T cells.