The role of the pre–T cell receptor (TCR) in lineage commitment to the γδ versus αβ lineage of T cells was addressed by analyzing TCRβ chain rearrangements in γδ T cells from wild-type and pre-TCR-deficient mice by single cell polymerase chain reaction. Results show that the pre-TCR selects against γδ T cells containing rearranged Vβ genes and that γδ T cell precursors but not γδ T cells express the pre-TCRα protein. Furthermore, pre-TCR-induced proliferation could not be detected in γδ T cells. We propose that the pre-TCR commits developing T cells to the αβ lineage by an instructive mechanism that has largely replaced an evolutionary more ancient stochastic mechanism of lineage commitment.