Numerous studies have highlighted the importance of high-affinity interactions between T cell receptors (TCRs) and their ligands in the selection of Foxp3(+) regulatory T cells (T reg cells). To determine the role of the TCR in directing T cells into the Foxp3(+) lineage, we generated transgenic (Tg) mice expressing TCRs from Foxp3(+) cells. Initial analyses of the TCR Tg mice crossed with RAG-deficient mice showed that the percentage of Foxp3(+) cells was very low. However, intrathymic injection and bone marrow chimera experiments showed a saturable increase of the Foxp3(+) population when T reg TCR Tg cells were present in low numbers. Furthermore, when analyzing whole thymi of T reg TCR Tg RAG-deficient mice, we found significantly more Foxp3(+) cells than in conventional T cell TCR Tg mice. Our results indicate that although the TCR has an instructive role in determining Foxp3 expression, selection of Foxp3(+) individual clones in the thymus is limited by a very small niche.