The aim of the present work was to determine the in vitro protoscolicidal effect of thymol against Echinococcus granulosus. Protoscoleces of E. granulosus were incubated with thymol at concentrations of 10, 5 and 1 mug/ml. The first signs of thymol-induced damage were observed between 1 and 4 days post-incubation. The maximum protoscolicidal effect was found with thymol at 10 microg/ml, viability reduced to 53.5+/-11.9% after 12 days of incubation. At day 42, viability was 11.5+/-15.3% and, reached 0% after 80 days. Thymol at concentrations of 5 and 1 microg/ml provoked a later protoscolicidal effect. Results of viability tests were consistent with the tissue damage observed at the ultrastructural level. The primary site of damage was the tegument of the parasite. The morphological changes included contraction of the soma region, formation of blebs on the tegument, rostellar disorganization, loss of hooks and destruction of microtriches. The data reported in this article demonstrate a clear in vitro effect of thymol against E. granulosus protoscoleces.