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Background: Ampelozizyphus amazonicus Ducke, a plant that is widely used by the population of the Amazonian region to prevent and treat malaria, was investigated in this work, which describes, for the first time, the antiplasmodial activity of its extracts and associates this activity with its isolated constituents. Methods: Different extracts with solvents of increasing polarity (hexane, chloroform, ethanol, and water) were obtained of the root bark. This procedure resulted in extracts that were characterized for their constituents. The cytotoxicity and activity of the extracts against Plasmodium berghei (schizontocidal activity, liver stage) and Plasmodium falciparum (3D7 and Dd2 strains, erythrocyte stage) were assessed in vitro. Results: Of the four extracts assayed against P. berghei, the chloroform extract showed the greatest activity, with an inhibitory concentration 50% (IC50) value of 30.1 μg/mL, followed by the aqueous extract (IC50 = 39.9 μg/mL). The chloroform extract exhibited the highest antiplasmodial activity in the erythrocyte stage of P. falciparum, with an IC50 value lower than 15 μg/mL. Fractionation of this more active extract led to the isolation and elucidation of pentacyclic triterpenes, lupeol, betulin and betulinic acid, which showed antiplasmodial activities with IC50 values ranging from 5.6 to 80.30 μM. The most active of these, betulinic acid, was further quantified in the extracts by high-performance liquid chromatography-photodiode array detector analyzes. The higher amount was found in the chloroform extract, which was the most active one against P. falciparum. Conclusion: The results obtained in this work may partly explain the popular intake of A. amazonicus as an antimalarial remedy in the Amazon region. [ABSTRACT FROM AUTHOR]