Ag-driven selection mediates effective T cell help and the development of Th cell memory in vivo. To analyze the dynamics of interclonal competition during the selection process in vivo, we use the I-Ek-restricted murine response to pigeon cytochrome c (PCC). The dominant PCC-specific clonotype expresses Valpha11Vbeta3 V regions with preferred sequence features in the third hypervariable regions (CDR3). In the current study we define and quantitatively monitor four subdominant PCC-specific clonotypes that express Valpha11 paired with non-Vbeta3 TCR beta-chains (Vbeta6, Vbeta8.1/8. 2, Vbeta8.3, and Vbeta14). The subdominant clonotypes emerge with similar dynamics to the dominant clonotype and together amount to similar numbers as the dominant clonotype in vivo. These subdominant clonotypes do not efficiently enter germinal centers, although they enter the memory compartment and rapidly re-emerge upon secondary challenge. Analysis of CDR3 diversity in the TCR alpha-chains identifies many preferred sequence features expressed by the dominant clonotype. These studies quantitatively demonstrate selection for diverse Th cells in vivo and highlight TCR alpha-chain dominance in Ag-driven selection for best fit.