Abstract The composition of the essential root oil of Peucedanum lancifolium Lange (A piaceae), growing in the Atlantic coast regions of France, North-West Spain und Portugal was investigated by chromatographic and spectroscopic methods. Forty five terpenoids, many of them of already known structure, and several aliphatic esters have been identified. Besides the main constituent, the irregular sesquiterpene alcohol trans-sesquilavandulol, representing 53.37% of the total oil, some hitherto unknown sesquiterpenoids with an irregular carbon skeleton were found. The possible chemotaxonomic value of the essential oil com position with regard to the close botanical relationship of Peucedanum lancifolium Lange and Peucedanum palustre (L .) M oench is briefly discussed. With the exception of these two species, trans-sesquilavandulol was not found in any other member of the genus Peucedanum.