Thujone is a toxic monoterpene produced in needles of western red cedar ( Thuja plicata Donn) and transferred to the soil with defoliation. Thujone is toxic at relatively low levels to a wide range of organisms. This study was conducted to determine whether thujone could be detected in the soil in a western red cedar stand in Denmark. Western red cedar needles and soil samples from the organic and mineral soil underneath were collected in Lille Bøgeskov forest near Sorø in Denmark. Thujone was extracted from the samples by steam distillation, followed by liquid–liquid extraction into hexane, and determined by gas chromatography. Needles sampled from a western red cedar tree at four heights between 1.9 and 17.8 m had concentrations of thujone in the range 2220–6550 mg kg -1 dry weight. The thujone content in the organic soil layer was in the range 170–1200 mg kg -1 dry weight, whereas only 1.2–3.0 µg kg -1 was found in the upper 2 cm of the mineral soil. The high amounts of thujone present in the forest floor may therefore have a significant effect on organisms in soils and fresh water streams in a western red cedar forest.