During positive selection of CD4(+), CD8(+) double positive (DP) thymocytes, expression of the tyrosine kinase Zap70 is subject to developmental regulation. Signalling downstream of T-cell receptor (TCR) induces Zap70 expression, forming a positive feedback circuit. Although previous studies show this circuit is required for generation of CD8 lineage cells, it is not known whether selection of CD4 T cells also depends on intact developmental regulation of Zap70. To address this, we analysed development of Class II-restricted thymocytes in mice lacking the Zap70 transcriptional circuitry. Rescue of Zap70 expression in Zap70(-/-) mice using a tetracycline-inducible Zap70 transgene, that is not subject to positive feedback by TCR signalling, restored positive selection of Class-II-restricted thymocytes. However, in conditions of static Zap70 expression, approximately half of selecting thymocytes failed to commit normally to the CD4 lineage. Instead, cells that failed to develop into CD4 T cells resembled CD8 lineage precursor DP thymocytes but failed to survive in vivo. Therefore, the Zap70 feedback circuit is essential to efficiently mediate the CD4 lineage differentiation programme in response to Class II selecting ligands.