Detailed systematic and comprehensive analysis of the essential oil of the leaves of Costus afer Ker–Grawl (Zingiberaceae) from the western part of Nigeria by capillary gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) led to the identiﬁcation of 27 compounds from 38 compounds in the essential oil. The results showed that sesquiterpenoids were the most abundant group of volatile compounds, with sesquilavandulyl acetate (17.0%) as the principal component. β-caryophyllene (12.3%) and Z,E-farnesol (9.9%) were also present in reasonable quantities. Investigation of the antimicrobial activity of the essential oil showed that it is inactive. Copyright © 2003 John Wiley & Sons, Ltd.