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Context: Piqueria trinervia Cav. (Asteraceae) is a plant species with a long history in traditional medicine to cure diarrhoea and other digestive disorders. Objective: The study investigates the antigiardial activity of piquerol, trinervinol, red oil and two fractions (F1 and F2) from P. trinervia. Materials and methods: P. trinervia was collected in the Ajusco in Mexico City. Aerial parts were ground and mixed with water to obtain the extract, which was treated with dichloromethane to isolate piquerol and trinervinol (P & T). Remnants were the red oil, fractions 1 and 2 (RO, F1 & F2). Trophozoites of Giardia intestinalis were treated with P, T, RO, F1 and F2 at different concentrations (0.78–200 μg/mL) for 48 h. Antigiardial activity was measured using the methylene blue reduction, and the cytotoxicity assayed on human fibroblasts and Vero cells by reduction of tetrazolium salts. Results: Trinervinol and piquerol showed antigiardial activity with an IC50 = 2.03 and 2.42 μg/mL, and IC90 = 13.03 and 8.74 μg/mL, respectively. The concentrations of trinervinol (CC50 = 590 μg/mL) and piquerol (CC50 = 501 μg/mL) were not cytotoxic to human fibroblasts. Conclusions: Compounds from P. trinervia showed antigiardial activity; to enhance this activity, piquerol and trinervinol can be chemically modified.