PROTECTIVE immunity against malaria is induced by vaccination of hosts with irradiation-attenuated sporozoites. This immunity is mediated in part by neutralizing antibodies that are directed mainly against the repeat domain of the circumsporozoite protein^1–4. Early experiments showed, however, that B-cell-depleted mice that are immunized with sporozoites can resist challenge, indicating that T-cell effector mechanisms may also have a role in protection⁵. This idea was supported by the recent observation that protective immunity also requires T-cells expressing the CDS antigen (CD8⁺ T cells) ^6,7, whose target is probably the developing liver-stage parasites^8–10. Moreover, an oral Salmonella vaccine that expresses the circumsporozoite protein is able to protect against murine malaria in the absence of antibodies¹¹. Here we report the identification of an epitope contained within amino acids 249–260 of the Plasmodium berghei circumsporozoite protein that is recognized by H–2Kd-restricted cytotoxic T cells^12,13. Passive transfer into mice of cytotoxic-T-cell clones that recognize this epitope conferred a high degree of protection against challenge. These results provide the first direct evidence that CD8⁺ T cells that are specific for a defined epitope can confer protection against a parasitic infection.