The extent of structural conservation of the Plasmodium falciparum sporozoite surface protein gene, STARP, recently characterized in the T9/96 clone, has been analyzed using the polymerase chain reaction. Results from Ivory Coast and Thai clones, field isolates originating from Brazil and Kenya and laboratory-maintained strains strongly suggest that this gene has a highly conserved structure throughout this species. This structure includes a complex repetitive central domain consisting of a mosaic region followed by tandem 45-amino acid-encoding (Rp45) and 10-amino acid-encoding (Rp10) repeat regions. Limited size variation in this domain appeared to result from highly localized duplication events in the Rp45 and Rp10 regions. No size variation was observed in the 5' and 3' coding non-repetitive regions, but minor size polymorphism was found in the single intron at the 5' end of the gene. No evidence was found of distinct families of polymorphic types, as has been observed with the blood-stage MSA-1, MSA-2 and S-antigens. The sequence of the STARP homologue in the phylogenetically close chimpanzee parasite, Plasmodium reichenowi, has also been elucidated and reveals high sequence conservation, although interesting differences were detected in the composition of the Rp10 region, known in P. falciparum to contain B- and T-cell epitopes. Finally, DNA hybridization reveals the presence in rodent malaria species of sequences containing homology to the STARP non-repetitive (though not the repetitive) regions, which would suggest that a similar, conserved gene may exist in these species.