Essential oils obtained by hydrodistillation of leaves from specimens of Ageratum conyzoides and Acritopappus confertus were investigated by GCâMS. The leaf essential oil of A. conyzoides consisted exclusively of the chromenes precocene I (95.4%) and II (4.5%). The essential oil of A. confertus consisted predominantly of monoterpenes, representing 81.0% of the total. The main component was myrcene (52.0%), with reasonable amounts of β-pinene (16.8%) and limonene (8.2%). The essential oil of A. conyzoides from the north-east of Brazil has been reported as the major source of precocene I to date. Copyright © 2005 John Wiley & Sons, Ltd.