the chemical composition of the essential oils obtained by hydrodistillation from the aerial parts of the tunisian hypericum perforatum and h. ericoides ssp. roberti was elucidated by a combination of gc and gc-ms analyses. the main constituents of the oil of h. perforatum were alpha-pinene (11.8%), alpha-ylangene (10.4%), germacrene-d (9.5%), n-octane (6.5%) and alpha-selinene (5.9%). the oil of h. ericoides ssp. roberti exhibited a higher amount of aliphatic and branched hydrocarbons and the main constituents were n-octane (29.1%), alpha-pinene (10.9%), pulegone (7.7%) and acetophenone (7%). both qualitative and quantitative differences were observed between the studied oils. this chemical variability seems likely to result from the genetic variability, since samples of both species were collected at the same location and processed under the same conditions.