Th17 CD4+ effector T cells play important roles in autoimmunity and responses to bacterial infections. The roles of cytokines in regulating the transcription factors STAT3 and RORγT in Th17 differentiation have been extensively studied. However, the role of T cell receptor (TCR) signaling in Th17 differentiation is less appreciated. To examine this issue, we evaluated IL17 production from T cells lacking Itk, a tyrosine kinase required for full TCR-induced activation of PLC-γ and downstream pathways. We find that Itk-/- CD4+T cells exhibit specific defects in the expression of IL-17A despite relatively normal expression of RORγT, RORα and the other Th17 cytokines, including the closely related IL-17F. Defects in IL-17A expression and differential regulation of IL-17A and IL-17F were also observed in vivo in Itk-/- mice challenged with an allergic asthma model. Although Itk-deficient cells have slightly depressed phosphorylation of STAT-3 in response to IL-6, expression of constitutively activated STAT-3 failed to rescue their IL-17A production. In contrast, expression of IL-17A could be rescued by pharmacologically-induced Ca2+ influx or by a constitutively active NFATc1. The effects of Itk on transcriptional complexes and chromatin at the IL-17 locus are under further investigation. Our results suggest that Itk specifically couples TCR signaling strength to IL-17A expression through NFATc1 and that TCR signaling pathways differentially influence Th17 cytokine production.