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After interviewing natives and migrants from the Amazon region of Brazil about plants traditionally used for treatment of malaria fever and/or liver disorders, we selected and identified 41 different species, including the native Bidens (Asteraceae). We have undertaken an antimalarial study of Bidens pilosa and other species of Bidens from abroad. The crude ethanol extracts (whole plant, leaves and roots) and the chloroform and butanol fractions from B. pilosa at concentrations of 50 μg/ml caused up to 90% inhibition of Plasmodium falciparum growth in vitro. In vivo the fractions caused partial reduction of Plasmodium berghei parasitemia in mice. The ethanol extracts from nine different Bidens species collected outside Brazil were tested, and seven inhibited parasite growth in vitro by 65–91%. As B. pilosa appears to be a promising antimalarial agent, we further characterized the substances responsible for such …