T lineage commitment occurs in a discrete, stage-specific manner during thymic ontogeny. Intrathymic precursor transfer experiments and the identification of CD4 ⁺ 8 ⁺ double-positive (DP), Vα14Jα18 natural T (iNKT) cells suggest that commitment to this lineage might occur at the DP stage. Nevertheless, this matter remains contentious because others failed to detect Vα14Jα18-positive iNKT cells that are CD4 ⁺ 8 ⁺ . In resolution to this issue, we demonstrate that retinoic acid receptor-related orphan receptor γ (RORγ) ^0/0 thymi, which accumulate immature single-positive (ISP) thymocytes that precede the DP stage, do not rearrange V α 14 -to- J α 18 gene segments, suggesting that this event occurs at a post-ISP stage. Mixed radiation bone marrow chimeras revealed that RORγ functions in an iNKT cell lineage-specific manner. Further, introgression of a Bcl-x _L transgene into RORγ ^0/0 mice, which promotes survival and permits secondary rearrangements of distal V α and J α gene segments at the DP stage, rescues V α 14 -to- J α 18 recombination. Similarly, introgression of a rearranged V α 14J α 18 transgene into RORγ ^0/0 mice results in functional iNKT cells. Thus, our data support the “T cell receptor-instructive (mainstream precursor) model” of iNKT cell lineage specification where V α 14 -to- J α 18 rearrangement, positive selection, and iNKT cell lineage commitment occur at or after the DP stage of ontogeny.