The essential oil composition of a population of Seseli tortuosum, collected on the coasts of Marina di Vecchiano (Pisa) Italy, was studied for the first time. The essence, obtained from leaves and flowering tops of the plant, was characterized mainly by the presence of myrcene (29.2%), -pinene (18.6%), β-pinene (13.2%) and limonene (10.6%). Among sesquiterpenes, only acorenone is quantitatively represented (6.3%). This is a rare sesquiterpene, isolated for the first time in Acorus calamus, quite uncommon in essential oils. Copyright © 2002 John Wiley & Sons, Ltd.