The choice between the αβ or γδ T cell fates is influenced by the production of functional, in-frame rearrangements of the TCR genes, but the mechanism that controls the lineage choice is not known. Here, we show that T cells that are heterozygous for a mutation of the Notch1 gene are more likely to develop as γδ T cells than as αβ T cells, implying that reduced Notch activity favors the γδ T cell fate over the αβ T cell fate. A constitutively activated form of Notch produces a reciprocal phenotype and induces thymocytes that have functional γδTCR gene rearrangements to adopt the αβ T cell fate. Our data indicate that Notch acts together with the newly formed T cell antigen receptor to direct the αβ versus γδ T cell lineage decision.