The essential oils from leaves of Baccharis latifolia and B. prunifolia collected in January 2006 were analyzed by GC/MS. The yields of oils extracted by hydrodistillation were 0.27 and 0.29% for B. latifolia and B. prunifolia, respectively. Sixteen (B. latifolia) and twenty nine (B. prunifolia) components were identified by comparison of their mass spectra with the Wiley GC-MS Library data and by their retention indices (RI). The identified products may be divided into four different groups: monoterpenes (9.0% B. latifolia; 43.9% B. prunifolia), oxygenated monoterpenes (0.8% B. latifolia; 5.4% B. prunifolia), sesquiterpenes (20.4% B. latifolia; 45.9% B. prunifolia) and oxygenated sesquiterpenes (69.8% B. latifolia; 1.9% B. prunifolia). The oils showed antibacterial activity only against Gram positive bacteria, with MIC values for Staphylococcus aureus (ATCC 25923) of 80 μg/mL (B. latifolia) and Enterococcus faecalis (ATCC 29212) of 90 μg/mL and 260 μg/mL (B latifolia and B. prunifolia, respectively).