A population-based study in Burkina Faso was conducted to determine, under conditions of natural exposure to malaria parasites, the prevalence rate and seasonal variation of antibodies to the repeated amino acid sequences of the Plasmodium falciparum ring-infected erythrocyte antigen (RESA) and the circumsporozoite (CS) protein, and the relationship of the presence of malaria parasites in the blood with the level of these antibodies. A random sample of households was selected and visited twice during 1988, in April just before the rainy season and in September at the end of the rainy season. Serological testing included an immunofluorescent assay for total anti-blood stage antibodies; a modified immunofluorescent assay on glutaraldehyde-fixed parasitized erythrocytes to detect antibodies to RESA; and an enzyme-linked immunosorbent assay (ELISA) using synthetic peptides representing repeat amino acid sequences of RESA [(EENV)5, (EENVEHDA)4, and (DDEHVEEPTVA)2] and CS protein [(PNAN5)]. In April, at the end of the dry season, 98% of the individuals had total P. falciparum antibodies, and 39% had anti-RESA antibodies. 32%, 82%, 80%, and 41% of the individuals had antibodies to the respective peptides. All prevalence rates increased with age. After the rains, in September, prevalence rates and levels of reactivity increased in each serological assay, except in the ELISA with (DDEHVEEPTVA)2. Presence of P. falciparum parasitaemia was inversely related to antibody response to the CS protein (PNAN)5 peptide but not to any other assay.