Activation of a cytotoxic T lymphocyte (CTL) response in an antigen-exposed lymph node involves a great diversity of encounters between naive CTL and APC that differ in both duration and quality. This broad spectrum of priming events instigates a complex blend of CTL developmental pathways. Using an experimental system that allows tight control over CTL priming, we have singled out defined priming events and analyzed the impact of the resulting instructional program on the effector and memory phases of the CTL response. As expected, prolonged antigenic stimulation induces potent CTL expansion, effector function and CTL memory. In contrast, CTL that have received suboptimal stimulation fail to undergo extensive expansion. Nevertheless these arrested CTL persist long term and acquire memory function. Thus, our data demonstrate that CTL memory can develop as a result of a suboptimal stimulation that causes arrested clonal expansion.