The ethyl acetate extract of the whole aerial parts of Varthemia iphionoides (Compositae) showed a pronounced antibacterial activity. The bioassay-guided fractionation of the ethyl acetate extract and further purification of the most antibacterially active fraction led to the isolation and identification of an antibacterial sesquiterpene, selina-4,11(13)-dien-3-on-12-oic acid. This compound exhibited potent antimicrobial activity against six bacterial species (Staphylococcus aureus, Bacillus subtilis, Micrococcus luteus, Escherichia coli, Bacillus cereus and Salmonella enteritides). The minimum inhibitory concentrations (MICs) of this compound which was determined by the agar dilution method ranged between 250 and 500 microg/ml.