The guaianolide sesquiterpene pyrethroidinin (1) and the corresponding ketone parishin A (2) were isolated from Tunacetum densum subsp. amuni (Asteraceae). The absolute stereochemistry of 1 was established as (lR, 3R, 6S, 7S, 10R) on the basis of the differential shielding in the 'H NMR spectra of the (-)-and (+)-0-methylmandelate esters. The structure, including absolute stereochemistry, of 2 was confirmed by oxidation of 1 with Jones' reagent.