AbstractThe composition of the hydrodistilled essential oils obtained from the aerial parts of Hypericum hyssopifolium subsp. elongatum var. elongatum and H. heterophyllum Vent. were analysed by means of GC and GC–MS, and 66 compounds were determined in total. The oils showed remarkable differences in chemical composition. The oil of H. hyssopifolium, which is rich in monoterpenes, consists primarily of α‐pinene (57.3%), β‐pinene (9.0%), limonene (6.2%) and α‐phellandrene (4.4%). The oil of H. heterophyllum was a complex mixture consisting mainly of sesquiterpenes (72.9% of the total oil). In this oil, isocaryophyllene (17.1%), α‐pinene (11.6%), δ‐cadinene (9.5%), γ‐muurolene (8.2%), n‐decane (5.8%), γ‐cadinene (5.5%) and β‐caryophyllene (4.5%) were found to be major constituents. The two essential oils were tested for antifungal activity using microbial growth inhibition assays in vitro against 10 agricultural pathogenic fungi, which consisted of five Fusarium species (F. oxysporum, F. culmorum, F. sambucinum, F. solani and F. acuminatum) and five anastomosis groups of Rhizoctonia solani (AG‐3, AG‐4, AG‐5, AG‐9 and AG‐11). In general, the oils showed moderate activity against several fungal species, viz F. acuminatum, AG‐5 and AG‐11. The most significant results were obtained against AG‐11 for H. heterophyllum oil. However, both oils increased the growth of some fungal species. In addition, the antifungal activity of 13 pure compounds identified as major components in the essential oils of the Hypericum species studied were determined using microbial growth inhibition assays against the 10 fungal species mentioned above. Among these compounds, both β‐caryophyllene oxide and α‐terpineol were inhibitory to the growth of all fungi. Copyright © 2003 John Wiley & Sons, Ltd.