The current therapy for leishmaniasis, which affects annually about 2 million people, is far from satisfactory. All available drugs require parenteral administration and are potentially toxic. Plant essential oils have been traditionally used in folk medicine and appear as valuable alternative source for chemotherapeutic compounds. In this study, we demonstrated the effect of essential oils from Cymbopogon citratus, Lippia sidoides, and Ocimum gratissimum on growth and ultrastructure of Leishmania chagasi promastigote forms. Steam distillation was used to isolate the essential oils, and their constituents were characterized by gas chromatography coupled to mass spectrometry and nuclear magnetic resonance. All essential oils showed in vitro inhibitory action on L. chagasi promastigotes growth in a dose-dependent way, with IC(50)/72 h of 45, 89, and 75 microg/mL for C. citratus, L. sidoides, and O. gratissimum, respectively. Drastic morphological alterations were observed in all essential oil-treated parasites, including cell swelling, accumulation of lipid droplets in the cytoplasm, and increase of acidocalcisome volume. Furthermore, aberrant-shaped cells with multi-septate body were observed by scanning electron microscopy, suggesting an additional effect on cytokinesis. Taken together, our data show that these essential oils affect the parasite viability being the C. citratus essential oil the most effective against L. chagasi.