The RNaseIII enzyme Dicer is required for mature microRNA production. Although extensive investigation has been carried out to determine the role of Dicer/miRNAs in the immune system, their function in mature CD8⁺ T cells has not been examined. We deleted Dicer in mature polyclonal and TCR transgenic CD8⁺ T cells using either tat-cre or the distal lck promoter, which drives cre expression after the stage of positive selection. Following antigenic challenge by a pathogen infection in vivo, Dicer-deleted CD8⁺ T cells failed to accumulate at the usual peak of the response. Surprisingly however, we found that deletion of Dicer in mature CD8⁺ T cells allowed them to respond more rapidly than control cells to TCR stimuli in vitro. In response to anti-CD3 plus anti-CD28 stimulation, Dicer-deleted T cells up-regulated CD69 faster and entered the first mitosis earlier than control T cells. In addition, activated Dicer^−/− cells failed to rapidly down-regulate CD69 when removed from the TCR stimulus. As a probable consequence of this sustained CD69 expression, Dicer^−/− T cells showed defective migration out of the central lymphoid organs in vivo. We identify miR-130/301, which are dramatically up-regulated following T-cell activation, as able to down-regulate CD69 expression via binding to a conserved site in the 3′UTR of CD69 mRNA. Thus, cellular functions dependent on Dicer expression are not required for the early steps in CD8⁺ T-cell activation, but are essential for their survival and accumulation.