To infer recent patterns of malaria transmission, we measured naturally acquired IgG antibodies to the conserved 19-kDa C-terminal region of the merozoite surface protein (MSP)-1 of both Plasmodium vivax (PvMSP-119) and Plasmodium falciparum (PfMSP-119) in remote malaria-exposed populations of the Amazon Basin. Community-based cross-sectional surveys were carried out between 2002 and 2003 in subjects of all age groups living along the margins of the Unini and Ja˙ rivers, Northwestern Brazil. We found high prevalence rates of IgG antibodies to PvMSP-119 (64.0 - 69.6%) and PfMSP-119 (51.6 - 52.0%), with significant differences in the proportion of subjects with antibodies to PvMSP-119 according to age, place of residence and habitual involvement in high-risk activities, defining some groups of highly exposed people who might be preferential targets of malaria control measures. In contrast, no risk factor other than age was significantly associated with seropositivity to PfMSP-119. Only 14.1% and 19.3% of the subjects tested for antibodies to PvMSP-119 and PfMSP-119 in consecutive surveys (142 - 203 days apart) seroconverted or had a three fold or higher increase in the levels of antibodies to these antigens. We discuss the extent to which serological data correlated with the classical malariometric indices and morbidity indicators measured in the studied population at the time of the seroprevalence surveys and highlight some limitations of serological data for epidemiological inference.