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A major unanswered question is what distinguishes the majority of activated CD8 T cells that die after an acute viral infection from the small fraction (5–10%) that survive to become long-lived memory cells. In this study we show that increased expression of the interleukin 7 receptor α-chain (IL-7Rα) identifies the effector CD8 T cells that will differentiate into memory cells. IL-7R^hi effector cells contained increased amounts of antiapoptotic molecules, and adoptive transfer of IL-7R^hi and IL-7R^lo effector cells showed that IL-7R^hi cells preferentially gave rise to memory cells that could persist and confer protective immunity. Thus, selective expression of IL-7R identifies memory cell precursors, and this marker may be useful in predicting the number of memory T cells generated after infection or immunization.