547.913.6 Several aromatic plants are used as herbal tea in Turkey, and one of the most commonly used plants for herbal tea is the genus Sideritis, which is widely grown, particularly in Aegean and Mediterranean areas, and represented by 46 species with high endemism (≅ 80%). Sideritis condensata Boiss. & Heldr. (Lamiaceae), known as Mountain Tea or Donkey Tea in Turkish, is widely distributed in the Western part of Anatolia [1].In previous studies, essential oil compositions of the species of S. condensata from different localities were reported [2, 3], and recently antioxidant and antibacterial activities of the extracts of the species have also been reported [4]. However, there is no study on the diterpenoid composition and other constituents of the species in the literature. We report herein a study of the diterpenoid constituents of Sideritis condensata and the insecticidal/acaricidal activity of the acetone extract and the pure compound linearol (1), which is a major diterpenoid in many species of Sideritis, against Bemicia tabaci, Lasioderma serricorne, Tetranychus urticae, Sitophilus granarius, Acanthoscelides obtectus, and Ephestia kuehniella. This is the first report on the diterpenoid constituents of S. condensata and insecticidal activity of the acetone extract of S.condensata and the kaurane diterpenoid linearol (1).From the Sideritis condensata extract, two kaurane, three iso-kaurane, and two epoxykaurane diterpenoids were isolated. They were identified as linearol (1), isolinearol (2), siderol (3), sideridiol (4), sideroxol (5), 7-acetylsideroxol, and candol B (7).Considering the chemotaxomic evaluation of Sideritis species, 46 species and 10 subspecies in Turkey and over 120 species in the Mediterranean area from the Caucasus to the Canary Islands have been reported [5]. The reported species from Turkey showed that their main constituents are kaurane diterpenoids. The species S. perfoliata and S. trojana were reported to have a single pimarane skeleton, and a labdane diterpene from S. argyrea was also reported. Eighty-one diterpenoids have been isolated from 16 species of Turkish Sideritis, including with this study, 77 of which have a kaurene skeleton with 31 distinct structures. The other four are labdane, pimarane, and beyerane. These results showed that 95% of species of Turkish Sideritis have diterpenoids with the kaurane skeleton [5][6][7][8][9][10][11][12]. However, species of Sideritis from the Western Mediterranean area and the Canary Islands contain diterpenoids with a greater diversity of structures such as ent-labdane [13], pimarane, and manoyl oxides [14][15] in addtition to kaurane diterpenoids. Six kaurane and seven labdane diterpenoids have been reported, isolated from Spanish S. chamaedryfolia [16]. Moreover, the presence of the new labdane and ten manoyl oxide diterpenoids in the ethanol extract of S. gomerae [13] from the Canary Islands was also reported. Additionally, Sideritis species, collected from Spain, have also incorporated bicyclic and tricyclic diterpenoids such as...