The aim of the present work was to determine the in vitro effect of Mentha piperita and Mentha pulegium essential oils against Echinococcus granulosus and to compare the effectiveness of both oils according to the exposure time and concentration. Although both treatments had a protoscolicidal effect, M. pulegium had a considerably stronger effect than M. piperita. Essential oil of M. pulegium produced dose- and time-dependent effects. Maximal protoscolicidal effect was observed after 12 days of incubation and reached 0% after 18 days. This lack of viability was proved during the determination of infectivity into mice. Essential oil of M. piperita produced only a time-dependent effect. At 24 days p.i., the viability of protoscoleces decreased to approximately 50%. Scanning and transmission electron microscopy (SEM and TEM) demonstrated the drug-induced ultrastructural damage. On the other hand, a loss of turgidity was detected in all M. pulegium-treated cysts respective of the drug concentration. There was a correlation between the intensity of damage and the concentration of the essential oil assayed. Studies by SEM revealed that the germinal layer of treated cysts lost the feature multicellular structure. M. pulegium essential oil showed piperitone oxide as main compound in their composition, and we suggest that this component could be responsible of the markedly anthelmintic effect detected. Our data suggest that essential oils of Mentha spp. can be a promising source of potential protoscolicidal agents. The isolation of active anthelmintic constituents is in progress and may lead to the discovery of compounds with improved therapeutic value.