Merozoite surface protein 1 (MSP 1) of Plasmodium falciparum has a major allelic dimorphism in the majority of its sequence, the origin and significance of which is obscure. Here, the cloning and sequencing of the msp1 gene from P. reichenowi (a chimpanzee parasite that is the nearest relative of P. falciparum) and P. gallinaceum (a malaria parasite of birds) is reported. P. reichenowi msp1 is most closely related to one allelic type (K1) of P. falciparum. The other P. falciparum major allelic type (MAD20) is very divergent from these sequences, although not as divergent as msp1 of P. gallinaceum. Assuming a date of 6 million years ago (mya) for the divergence of the P. falciparum K1 and the P. reichenowi msp1 genes (on the basis of previous estimates for these parasite species as well as host divergence times), the most recent common ancestor of the dimorphic region of msp1 would date to approximately 27mya. Thus, the P. falciparum msp1 dimorphism is confirmed as one of the oldest polymorphisms known with the exception of self-incompatibility S genes in Solanaceae. In contrast with the major allelic dimorphism, the polymorphisms present in the relatively conserved C terminus of P. falciparum msp1 appear to have arisen since the divergence of the P. falciparum and P. reichenowi msp1 genes.