The Syk/ZAP-70 family of protein tyrosine kinases is indispensable for normal lymphoid development. Syk is necessary for the development of B cells and epithelial γδ T cells, whereas ZAP-70 is essential for the normal development of T cells and TCR signaling. In this study, we show that although development of the αβ lineage was arrested in the thymus, CD3-positive T cells, primarily of the γδ lineage, were present in the lymph nodes of mice lacking ZAP-70. Moreover, in the absence of ZAP-70, dendritic epidermal T cells were fewer in number and of abnormal morphology, and intestinal intraepithelial lymphocytes, normally containing a large proportion of γδ T cells, were markedly reduced. These data suggest that γδ T cells show a variable dependence upon ZAP-70 for their development. Biochemical analyses of thymocytes revealed a lack of basal ζ-chain tyrosine phosphorylation. However, several other substrates were inducibly tyrosine phosphorylated following TCR stimulation. Thus, TCR-mediated signaling in ZAP-70-deficient thymocytes is only partially impaired. These studies suggest that Syk compensates only partially for the loss of ZAP-70, and that there is an absolute requirement of ZAP-70 for αβ T cells and epithelial γδ T cells, but not for some γδ T cells in peripheral lymphoid tissues.