Apical membrane antigen 1 is a candidate vaccine component for malaria. It is encoded by a single copy gene and has been characterised in a number of malaria species as either an 83-kDa de novo product (Plasmodium falciparum; Pf AMA-1) or a 66-kDa product (all other species). All members of the AMA-1 family are expressed during merozoite formation in maturing schizonts and are initially routed to the rhoptries. Processed forms may subsequently be associated with the merozoite surface. Because of the unique occurrence of the 83-kDa form in P. falciparum we were interested to determine whether the phylogenetically closely related chimpanzee malaria Plasmodium reichenowi shared characteristics with Pf AMA-1. Here we show that the molecular structure, the localisation and processing are similar to that of Pf AMA-1 and that in vitro growth inhibitory mAbs reactive with Pf AMA-1 also inhibit P. reichenowi growth in an in vitro assay. Polymorphism in the 83-kDa AMA-1 family was analysed through comparison of Pr ama-1 with Pf ama-1 alleles, which showed the most significant evidence for selection maintaining polymorphism in Domains I-III of AMA-1 in P. falciparum. The most substantial divergence between Pr AMA-1 and Pf AMA-1 sequences was in the N-terminal region unique to the 83-kDa form of AMA-1. It was confirmed that the specific Pr ama-1-type allele was not present among P. falciparum parasites in an African population, and an allele coding for lysine at amino acid 187 was uniquely associated with field isolates in this population.