The MeOH extract of the leaves of rosemary (Rosmarinus officinalis L.) completely inhibited the motility of cultured epimastigotes of Trypanosoma cruzi at the concentration of 2 mg/ml after 2 h of incubation. Activity-guided fractionation of the MeOH extract has resulted in the isolation of three triterpene acids, betulinic, oleanolic and ursolic acids. Ursolic acid stopped the movement of all T. cruzi epimastigotes at the minimum concentration (MC(100)) of 40 micro g/ml (88 micro M) after 48 h of incubation. Oleanolic acid was less active (MC(100): 250 micro g/ml, 550 micro M) and betulinic acid was practically inactive.