Naturally arising CD25⁺ CD4⁺ regulatory T cells (T_R) play an important role in the prevention of autoimmunity. TCR specificity is thought to play a critical role in T_R development and function, but the repertoire and specificity of T_R TCRs remain largely unknown. We find by sequencing of TRAV14 (Vα2) TCRα chains associated with a transgenic TCRβ chain that the T_R and CD25⁻ CD4⁺ TCR repertoires are similarly diverse, yet only partially overlapping. Retroviral expression of TCRα genes in TCR transgenic RAG-deficient T cells revealed that a high frequency of TCRs derived from CD25⁺ but not CD25⁻ CD4⁺ T cells confers the ability to rapidly expand upon transfer into a lymphopenic host. Thus, these data show that a large proportion of naturally arising T_R have substantially more efficient interactions with MHC class II bound peptides from the peripheral self than CD25⁻ T cells.