The Syk protein tyrosine kinase (PTK) is essential for B, but not T or NK, cell development, although certain T cell subsets (ie, γδ T cells of intestine and skin) appear to be dependent on Syk. In this report, we have re-evaluated the role of Syk in T cell development in hematopoietic chimeras generated by using Syk-deficient fetal liver hematopoietic stem cells (FL-HSC). We found that Syk−/− FL-HSC were vastly inferior to wild-type FL-HSC in reconstituting T cell development in recombinant-activating gene 2 (RAG2)-deficient mice, identifying an unexpected and nonredundant role for Syk in this process. This novel function of Syk in T cell development was mapped to the CD44− CD25+ stage. According to previous reports, development of intestinal γδ T cells was arrested in Syk−/−→ RAG2−/− chimeras. In striking contrast, when hosts were the newly established alymphoid RAG2× common cytokine receptor γ-chain (RAG2/γ c) mice, Syk−/− chimeras developed intestinal γδ T cells as well as other T cell subsets (including αβ T cells, NK1. 1+ αβ T cells, and splenic and thymic γδ T cells). However, all Syk-deficient T cell subsets were reduced in number, reaching about 25–50% of controls. These results attest to the utility of chimeric mice generated in a low competitive hematopoietic environment to evaluate more accurately the impact of lethal mutations on lymphoid development. Furthermore, they suggest that Syk intervenes in early T cell development independently of ZAP-70, and demonstrate that Syk is not essential for the intestinal γδ T cell lineage to develop.