The essential oils of the leaves of female (0.13%) and male (0.45%) specimens of Baccharis semiserrata collected in the state of Paraná, Brazil, were analyzed in detail using GC-FID and GC-MS techniques. Both samples have low monoterpene content and present the sesquiterpene spathulenol as their main component, reaching 50.75% in the oil originating from the male specimen and 42.65% in that of the female specimen. Other major sesquiterpenes found were (female-male): aromadendrene (9.61-2.33%), α-muurolene (8.88-2.38%) and δ-cadinene (9.52-3.93%). These data put this plant species forward as potential spathulenol source.