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 CD44hi, 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.