Chaiya and Tha Chana districts of Surat Thani province are located in the areas with high levels of equivalent uranium at the ground surface, which have been identified as sources of radon. A survey measurement of indoor radon concentrations was carried out in 248 houses, using CR-39 detectors in closed cups. The geometric mean of indoor radon concentrations in Chaiya and Tha Chana districts were 26 ± 2 Bq-rn”3 and 30 ± 2 Bq*nr3, respectively. Although the minimum radon concentration was 4 Bq«m”3 in both locations, the maximum radon concentration was found to be 159 Bq«m’3 in Tha Chana district, while it was 88 Bq»m”3 in Chaiya district. The level of radon concentrations above the action level (148 Bq*m 3) recommended by the United States Environmental Protection Agency was only found in two houses, which accounted for 1% of the total buildings surveyed in this present study. The majority of houses, which accounted for 94% of the total buildings surveyed, showed the radon concentration below the action level. As these houses had access to air flow during the daytime through open doors and windows, it is likely that such ventilation was sufficient to keep radon at a low concentration.