Giardia lamblia is one of the most important worldwide causes of intestinal infections produced by protozoa. Current therapy for giardiasis is unsatisfactory due to high incidence of undesirable side effects and significant failure in clearing parasites from the gastrointestinal tract. In the search for new therapeutic agents, we report on the effect of several essential oils on G. lamblia growth. Among eighteen tested essential oils, those with phenolic compositions were the most active, particularly if containing high contents of carvacrol, such as Thymbra capitata and Origanum virens (IC50 values of 71 and 85 microg x mL(-1), respectively). The oils from Syzygium aromaticum and Thymus zygis subsp. sylvestris (IC50 values from 100 to 200 microg x mL(-1)), as well as, those from Mentha x piperita and Lippia graveolens (IC50 values over 200 microg x mL(-1)) were less active. Results support the concept that several essential oils or some of their constituents may be useful in the clinical management of Giardia infections.