Two cross-sectional surveys of 954 persons in Asembo Bay and Got Nyabondo, western Kenya, were performed in August–September 1986, after long rains, and in February–March 1987, after a comparatively dry season. Serologic testing was performed using an ELISA with synthetic peptides representing repeat amino acid sequences of the Plasmodium falciparum ring-infected erythrocyte surface antigen (RESA), (EENV)5, (EENVEHDA)4, and (DDEHVEEPTVA)2 and repeat sequences (PNAN)5 and (NAAG)5 of the P. falciparum and P. malariae circumsporozoite proteins. In 1986, 45%, 73%, 72%, 85%, and 59% of the persons in Asembo Bay had antibodies to the respective peptides. In Got Nyabondo, the rates were 44%, 67%, 56%, 36%, and 41%, respectively. All positivity rates increased with age. When next determined in 1987, the positivity rates and levels of reactivity were generally unchanged in Asembo Bay, but were decreased in Got Nyabondo.