Essential oil from leaves of Ageratina jahnii (B.L.Rob.) R. M. King & H. Rob. and Ageratina pichinchensis (Kunth) R. M. King & H. Rob (Asteraceae) collected in January 2010 were analyzed by GC/MS. Oils extracted by hydrodistillation yielded 0.50% and 0.43 % w/v, respectively. Fifteen and twenty five components were identified by comparison of their mass spectra with the Wiley GC-MS Library data and by their retention indices (RI). The major components identified in A. jahnii were β-myrcene (37.6 %), α-pinene (17.1 %), limonene (8.8 %) and pentacosane (9.2 %) while for A. pichinchensis 8,9epoxythymyl isobutyrate (20.2 %), germacrene-D (19.8 %), thymyl isobutyrate (10.8 %), eupatoriochromene (6.5 %) and encecalol (5.9 %) were observed as main compounds. This is the first report regarding the essential oil composition and antibacterial activity of the essential oil of A. jahnii.