Abstract The essential oil of Ageratum conyzoides from S. Tomé and Príncipe was investigated for the first time. The oil was obtained by hydrodistillation of the aerial parts, and subsequently analyzed by GC, GC/MS and 13C-NMR. The oil was characterized by the presence of high percentages of precocene I (34.4%) and β-caryophyllene (24.6%), as well as small amounts of precocene II. Using the disk diffusion method, the oil of A. conyzoides showed weak activity against Staphylococcus aureus, S. epidermidis, Streptococcus faecalis, Proteus vulgaris and Cladosporium cladosporioides, and was inactive against Escherichia coli, Candida albicans, Cryptococcus neoformans, Aspergillus niger and A. fumigatus. Key Word Index: Ageratum conyzoides AsteraceaeFiá-malé-homemessential oil compositionprecocene Iβ-caryophylleneantimicrobial activity