The chemical composition and antibacterial activity of the essential oil from the leaves of Ageratina pentlandiana (DC.) R.M. King & H. Rob. grown in Cuzco (Peru) was studied. One hundred and fifteen compounds were identified in the essential oil. The essential oil was dominated by phenyl derivative constituents (39.9%), including 2,5-dimethoxy-p-cymene (20.9%), thymol methyl ether (9.9%) and 2,5-dimethoxy-p-cymenene (8.2%) as the most abundant constituents identified in leaves oil. The antibacterial activity was studied by the disk diffusion method and minimal inhibitory concentration and minimal bactericide concentration were determined against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Salmonella tiphymurium. The essential oil showed a marked antibacterial activity, especially against S. aureus and B. subtilis.