The essential oil from the leaves of Baccharis trinervis (Lam.) Pers., collected in May 2006, was analyzed by GC/MS. A yield of 0.2% oil was obtained by hydrodistillation. Forty three components were identified by comparison of their mass spectra with those in the Wiley GC-MS Library data base. The major components were germacrene D (20.4%), limonene (15.4%), δ-cadinene (5.2%), β-caryophyllene (4.8%), α-pinene (4.5%) and bicyclogermacrene (4.0%). The essential oil showed antibacterial activity against the important human pathogenic Gram positive bacteria Staphylococcus aureus ATCC 25923 and Enterococcus faecalis ATCC 29212, with MIC values of 80 and 200 μg/mL, respectively.