The essential oil from aerial parts of Tagetes terniflora HBK. (Asteraceae) was obtained by hydrodistillation. The oil content was 0.15% (v/w), on a fresh weight basis. The oil composition was analysed by GC and GC/MS. Sixty-five compounds were identified accounting for 96.1% of the oil. Tagetones (22.4%), ocimenes (20.6%) and ocimenones (15.9%) predominated in the oil, with (E)-β-ocimene (12.8%) and trans-tagetone (10.7%) as the main compounds. The oil contained considerable amounts of limonene (3.9%), isomenthone (3.1%), and of spathulenol (2.7%) in sesquiterpenoidic fraction. cis-(4.1%) and trans-Anethole (2.1%) are the main constituent of the phenylpropanoid group. A preliminary in vitro toxicity test, carried out with Artemia salina, showed an ED50 value of the essential oil of 3.16 μg/ml. Copyright © 2004 John Wiley & Sons, Ltd.