In the present work, an endemic species Prangos heyniae collected in four locations from Turkey was subjected to hydrodistillation in Clevenger type apparatus to obtain the essential oils (E01-4). The gas chromatography/mass spectrometry (GC/MS) and gas chromatography-flame ionization detector (GC/FID) analyses showed that the EOs were rich in sesquiterpenes, germacrene D (10.3-12.1%), beta-bisabolene (14.4%), kessane (26.9%), germacrene B (8.2%), elemol (3.4-46.9%), beta-bisabolenal (14-70.7%), beta-bisabolenol (8.4%) and an eudesmane type sesquiterpene (1) (16.1%) with [M(+)218]. This unidentified compound (1) was isolated in a rapid one-step manner with >95.0% purity using Preparative Capillary Gas Chromatography (PCGC) with an HP Innowax column connected to a Preparative Fraction Collector (PFC) system. Structure determination was accomplished from 1D- and 2D-NMR spectroscopic data which determined a new eudesmane type sesquiterpene, 3,7(11)-eudesmadien-2-one (1). Using a biting deterrent bioassay, the mean proportion not biting (PNB) values of the P. heyniae E01-4 were 0.88 for E01 and 0.80 for E02 which were similar to the positive control DEET (N, N-diethyl-3-methylbenzamide). The E03 and E04 had lower PNB values of 0.64 and 0.44, respectively. P. heyniae E01-4 showed good larvicidal activity at 125 and 62.5 ppm whereas E01-3 were slightly less effective at the dose of 31.25 ppm and E04 was not active at 31.25 ppm against 1st instar Aedes aegypti.