The composition of the essential oils of Bystropogon plumosus, B. origanifolius var. palmensis, B. wildpretii, B. maderensis, and B. canariensis var. smithianus were studied by GLC and GLC/MS. The first four species provide relatively similar oils characterized by monoterpene ketones as the main constituents while the last species is quite different with main constituents consisting of mono- and sesquiterpene hydrocarbons. In B. plumosus the ontogenetic variation of the monoterpene ketones was studied for a period of four years. During this time the behaviour of pulegone was inversely correlated with that of menthone and isomenthone. Pulegone was the most active compound of the monoterpene ketones in antimicrobial tests using three species of bacteria and fungi, respectively.