The Tec family tyrosine kinases, Itk and Rlk, are expressed in thymocytes and peripheral T cells and regulate thresholds of T cell receptor signaling. Yet little is known about the specific role of Itk- and Rlk-dependent signals in CD8⁺ T cell maturation. We show here that Itk^−/− and Rlk^−/−Itk^−/− mice were nearly devoid of conventional CD8⁺ T cells and, instead, contained a large population of CD8⁺ T cells that bear striking similarity to lineages of innate lymphocytes. Itk^−/− and Rlk^−/−Itk^−/− CD8⁺ thymocytes and T cells were CD44^hi, CD122⁺, and NK1.1⁺; were able to produce interferon-γ directly ex vivo; and were dependent on interleukin-15. Itk^−/− and Rlk^−/−Itk^−/− CD8⁺ thymocytes expressed abundant transcripts for the T box transcription factor, eomesodermin, correlating with their phenotype and function. These data indicate a critical role for Itk and Rlk in conventional CD8⁺ T cell development in the thymus.