A subset of CD8 T cells in normal mice, expressing high levels of activation markers such as CD44, shares many properties with antigen-specific memory CD8 T cells. Homeostasis of CD44^high CD8 T cells depends upon cytokines such as interleukin-15 (IL-15); however, the downstream signaling pathways regulating IL-15–dependent homeostatic proliferation are poorly defined. Surprisingly, we show here that haploinsufficiency of the protooncogene c-myc leads to a highly selective decrease in CD44^high CD8 T cells in mice. Although steady-state proliferation and survival of CD44^high CD8 T cells appeared not to be dependent on c-Myc, homeostatic proliferation of c-myc^+/– CD44^high CD8 T cells in lymphopenic hosts was strongly reduced, and the residual homeostatic proliferation of these cells appeared to occur independently of IL-15. Moreover, c-myc^+/– CD44^high CD8 T cells responded very poorly to purified IL-15 in vitro. Backcrossing of c-myc^+/– mice to IL-15^–/– mice revealed that the number of CD44^high CD8 T cells decreased in an additive fashion in mice heterozygous for c-myc and IL-15. Finally homeostatic proliferation of antigen-specific memory CD44^high CD8 T cells was also impaired in c-myc^+/– mice. Collectively, our data identify c-Myc as a novel downstream component of the IL-15–dependent pathway controlling homeostatic proliferation of memory CD44^high CD8 T cells.