The ethnopharmacology for treatment of parasitic infections facilitates and directs the search for new chemical entities. In this direction, this study evaluated the cytotoxicity in vitro, against trophozoites of Giardia lamblia, of aqueous extracts of leaves Achyrocline satureioides (Lam.) DC., barks of Eugenia uniflora L., aerial parts of Foeniculum vulgare Miller, and barks of Psidium guajava L. These plants are traditionally used for the treatment of diarrhea by the indigenous population Mbyá-Guarani, located at the Lomba do Pinheiro, Porto Alegre, Rio Grande do Sul, Brazil. The in vitro susceptibility qualitative analysis of G. lamblia to aqueous extracts was performed in serial dilutions from 2.5 to 0.02 mg/mL. Results revealed the minimal inhibitory concentrations: 0.313 mg/mL for A. satureioides and E. uniflora, 0.02 mg/mL for P. guajava, and F. vulgare did not present any cytotoxic effect. Quantitative assays of viable trophozoites, showed that A. satureioides presented the highest cytotoxic effect (93.5%), followed by P. guajava (82.2%), and E. uniflora (67.3%). Indigenous Guarani use mainly A. satureioides for the treatment of diarrhea, revealing the conformity with results obtained in vitro. Bioguided assays are necessary to identify the compounds responsible for the activity of the aqueous extract of A. satureioides.