The gene for the circumsporozoite (CS) protein of Plasmodium falciparum has been cloned and its nucleotide sequence determined. The gene encodes a protein of 412 amino acids as deduced from the nucleotide sequence. The protein contains 41 tandem repeats of a tetrapeptide, 37 of which are Asn-Ala-Asn-Pro and four of which are Asn-Val-Asp-Pro. Monoclonal antibodies against the CS protein of Plasmodium falciparum were inhibited from binding to the protein by synthetic peptides of the repeat sequence. The CS protein of Plasmodium falciparum and the CS protein of a simian malaria parasite, Plasmodium knowlesi, have two regions of homology, one of which is present on either side of the repeat. One region contains 12 of 13 identical amino acids. Within the nucleotide sequence of this region, 25 of 27 nucleotides are conserved. The conservation of these regions in parasites widely separated in evolution suggests that they may have a function such as binding to liver cells and may represent an invariant target for immunity.