Eryngium vesiculosum Labill. is seasonally heterophyllous. The essential oils of its aerial parts gathered during summer and winter from New South Wales (Australia) have been examined by analytical gas chromatography and gas chromatography/mass spectrometry. We have found quantitative but not qualitative differences between the two samples. The principal compounds of the winter leaves were identified as ß-caryophyllene (20.3%), germacrene D (19.2%) and a-humulene (8.8%), while the summer leaves (SL) contained bicyclogermacrene (22.2%), ß-caryophyllene (15.6%), germacrene D (15.8%) and a-humulene (8.1%) as major constituents. The amount of bicyclogermacrene changes by a factor of ten, ranging from 2.4% in winter leaves to being the principal compound (22.2%) in summer leaves. The oil of the flowers and fruits contained almost the same composition as the summer leaves. This is the first comparative study on the chemistry of both kinds of leaves of E. vesiculosum .