During T-cell development, thymocytes with intermediate avidity for antigen-MHC complexes are positively selected and then differentiate into functional cytotoxic and helper T cells. This process is controlled by signalling from the T-cell receptor (TCR). Here, we show that the c-Myb transcription factor is a critical downstream regulator of positive selection, promoting the development of helper T cells and blocking the development of cytotoxic T cells. A gain-of-function c-Myb transgene stops development of cytotoxic T cells, instead causing accumulation of a precursor population. Conversely, loss of c-Myb in selecting cells results in significantly fewer helper T cells. In c-Myb-null thymocytes, Gata3, a critical inducer of T-helper cell fate, is not upregulated in response to T-cell receptor signaling, following selection. We show that Gata3 is a direct target of c-Myb, and propose that c-Myb is an important regulator of Gata3, required for transduction of the T-cell receptor signal for subsequent helper cell lineage differentiation.