Vγ9Vδ2 T cells are a major human bloodγδT cell population thatrespond in a T cell receptor (TCR)-dependent manner to phos-phoantigens which are generated by a variety of microorganisms.It is not clear how Vγ9Vδ2 T cells react toward the sudden micro-bial exposure early after birth. We found that human Vγ9Vδ2T cells with a public/shared fetal-derived TCR repertoire expandedwithin 10 wk postpartum. Such an expansion was not observed innon-Vγ9Vδ2γδT cells, which possessed a private TCR repertoire.Furthermore, only the Vγ9Vδ2 T cells differentiated into potentcytotoxic effector cells by 10 wk of age, despite their fetal origin.Both the expansion of public fetal Vγ9Vδ2 T cells and their func-tional differentiation were not affected by newborn vaccinationwith the phosphoantigen-containing bacillus Calmette–Guérin(BCG) vaccine. These findings suggest a strong and early primingof the public fetal-derived Vγ9Vδ2 T cells promptly after birth,likely upon environmental phosphoantigen exposure.