Loss of interleukin (IL)-7 or the IL-7 receptor alpha (IL-7Rα, CD127) results in severe immunodeficiencies in mice and humans. To more precisely identify signals governing IL-7 function in vivo, we have disrupted the IL-7Rα Y449XXM motif in mice by knock-in mutagenesis (IL-7Rα^449F). Thymic precursors were reduced in number in IL-7Rα^449F mice, but in marked contrast to IL-7Rα^−/− knockout mice, thymocytes and peripheral T cells developed normally. Strikingly, Listeria infection revealed that CD4 and CD8 T cells had different requirements for IL-7Rα signals. CD4 T cells failed to mount a primary response, but despite normal CD8 primary responses, maintenance of CD8 memory was impaired in IL-7Rα^449F mice. Furthermore, we show that Bcl-2 is IL-7Rα Y449 independent and insufficient for IL-7–mediated maintenance of CD8 memory.