Achyrocline satureioides extracts were tested in vitro against the growth of Paenibacillus larvae. Four different extracts were obtained by liquid-liquid extraction from an aqueous-ethyl alcohol macerate of the aerial parts of the plant. The biological activity was tested by the broth microdilution technique. Hexane extract showed the highest activity (minimum inhibitory concentration = 0.060 +/- 0.037 mg/mL). Transmission electron microscopy experiments showed that the main effect exerted by the hexane extract on the cell was at the cellular membrane level. The hexane extract was analyzed by thin-layer chromatography, and the activity of its components was tested by bioautography. Four growth inhibition zones were observed in the bioautographic experiments (using hexane-acetone (7:3) as mobile phase), with that at Rf = 0.57 showing the largest zone of inhibition. High-performance liquid chromatographic experiments, using ultraviolet and electrospray ionization coupled to tandem mass spectrometric detection, showed the presence of one compound with a m/z ratio of 442, which may be related to phloroglucinols alpha-pyrone compounds recently discovered. The high antibacterial activity of the hexane extract and of the isolated compound determined in this work may be useful for the development of future new alternatives for the treatment of American foulbrood.