The Foxp3-expressing subset of regulatory CD4+ T cells have defined Ag specificity and play essential roles in maintaining peripheral tolerance by suppressing the activation of self-reactive T cells. Similarly, during chronic infection, pathogen-specific Foxp3-expressing CD4+ T cells expand and actively suppress pathogen-specific effector T cells. Herein, we used MHC class II tetramers and Foxp3gfp knockin mice to track the kinetics and magnitude whereby pathogen-specific Foxp3+CD4 + and Foxp3+CD4+ cells are primed and expand after acute infection with recombinant Listeria monocytogenes (Lm) expressing the non-"self"-Ag 2W1S52-68. We demonstrate that Lm infection selectively primes proliferation, expansion, and subsequent contraction of Lm-specific Foxp3- effector CD4+ cells, while the numbers of Lm-specific Foxp3+CD4+ regulatory cells remain essentially unchanged. In sharp contrast, purified 2W1S 52-68 peptide primes coordinated expansion of both Foxp3+ regulatory and Foxp3- effector T cells with the same Ag specificity. Taken together, these results indicate selective priming and expansion of Foxp3- CD4 T cells is a distinguishing feature for acute bacterial infection.