SummaryAntibodies to two peptides DDAAD and (NANP)40representing the repetitive sequence of circumsporozoite antigens (CS protein) ofP. vivaxandP. falciparumrespectively were measured in a cohort of 149 and 107 individuals respectively at four, 6 monthly blood surveys performed on residents of Kataragama, aP. vivaxmalaria endemic region in southern Sri Lanka. The prevalence of antibodies to the CS protein of both species was relatively low being less than 20% to either peptide in the population as a whole, this being consistent with the low entomological inoculation rates in the area. A marked age related prevalence pattern was evident, with the prevalence of antibodies increasing with age to reach between 25 to 30% in the 25–50 year age group in bothP. vivaxandP.falciparum. The population had had a life long exposure toP. vivaxmalaria but not toP.falciparum, an epidemic of which occurred in this region a few months prior to the beginning of this study. Nevertheless, the age‐related prevalence of these antibodies was identical with respect to the two species. This suggests that the age‐related prevalence pattern reflected differences in inoculation rates between the age groups due to differences in exposure to inoculation rather than an age acquired response resulting from a cumulative experience over several years. An analysis of antibody prevalence in individuals showed first, that sporozoite inoculations must have been clustered rather than homogeneously distributed in the population and secondly, that sero‐conversion did not correlate with malaria infections in these indi