In the framework of the correlation between chemotaxonomy and chemical analysis studies, the chemical composition of the essential oils of four varieties of Thuja species cultivated in Poland - T. occidentalis 'globosa', T. occidentalis 'aurea', T. plicata and T. plicata 'gracialis' - were investigated by GC and GC-MS. Thirty-one compounds were identified from T. occidentalis 'globosa', representing 96.92% of the total oil; twenty-seven from T. occidentalis 'aurea' (94.34%); thirty-one from T. plicata (94.75%); and thirty compounds from T. plicata 'gracialis' (96.36%). The main constituents in all samples were the monoterpene ketones alpha- and beta-thujone, fenchone and sabinene, as well as the diterpenes beyerene and rimuene. The chemosystematic value of the total ketone content of all samples (which varied from 54.30-69.18%) has been discussed and investigated. The constituents, beyerene and the mixture of alpha- and beta-thujone, were isolated from the oils and tested against six Gram-positive and -negative bacteria and three pathogenic fungi. The oils of the two T. plicata species exhibited significant antimicrobial activity, while the mixture of alpha- and beta-thujone showed very strong activity as well.