This study presents new data on the baseline concentrations of Thorium over the Derekoy-Yazir (Aglasun-Burdur) volcanic area. Portable gamma-ray spectrometer was used for natural thorium mapping. In situ measurements were made in the field, in the area of 7.5 km(2) at 165 points. Variations in the Th concentration in the surficial environment of the Derekoy-Yazir region appear to be related to bedrock lithology. The measured thorium concentration varies between 0.68-36 ppm, in the studied area. The highest concentration values were obtained from volcanic rocks. The outcropping volcanic rocks in the region are Pliocene alkaline basalts.