Abstract The essential oil obtained by hydrodistillation of the aerial parts of Helichrysum forsskahlii, endemic to southern Saudi Arabia, was analyzed by GC/MS. Eighty-one compounds, representing more than 85.9% of the oil, were identified with selina-5,11-diene (45.3%), Î´-3-carene (7.8%), 1,8-cineole (4.2%) and Î²-caryophyllene (4.9%) as main constituents. The major component, selina-5, 11-diene, being not previously isolated from natural sources, was isolated by column chromatography and its identity confirmed by 1 and 2D NMR spectral analysis. The oil showed only a very weak activity against Mycobacterium smegmatis, with no activity against Gram-positive and Gram-negative bacteria or Candida test organisms. Likewise, selina-5, 11-diene showed only a weak activity against M. intracellulare.