Results derived from in situ gamma-ray spectrometry studies on geological units of the Isparta plain, situated in the foothills of the Golcuk volcano, are presented in this study. Determination of the natural radioactivity and radionuclide concentration has been carried out using 512 channel thallium-activated sodium iodide [NaI(TI)] gamma-ray detectors with crystal size 2aEuro(3) x 2aEuro(3) produced from GF Instrument Inc. Measurements were performed on 341 site covering volcanic and non-volcanic geological units in the area approximately 256 km(2). From the measured gamma-ray spectra, high activity concentration (K-40, U-238, Th-232) and air absorbed dose rates were found to be about 1,959.38, 443.48, 408.63 Bq/kg and 526 nGy/h in the volcanic units. In the residential area and its vicinity, average concentration value of K-40, U-238, Th-232 and absorbed dose rates were determined as about 1,387.63, 239.16, 234.34 Bq/kg and 350 nGy/h, respectively. Annual effective dose was found as 429.53 mu Sv/year in this area while annual effective doses were varying from 208 to 522.80 mu Sv/year in the study area. Contribution (in %) to the background radioactivity level emanated from about 17.58 % K-40, 35.73 % U-238, 46.69 % Th-232 in volcanic and materials composed of volcanic units.