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The results of 222 Rn and 226 Ra activity measurements in drinkable water supplies of the Thu Duc region in Ho Chi Minh City, Vietnam, are presented in this paper. The measurements were performed using a RAD 7 radon detector manufactured by Durridge Company, Inc. Mean concentrations of 222 Rn and 226 Ra were found to be 0.11±0.01 Bq l −1 and 0.11±0.02 Bq l −1 in 14 drinking water samples. They are 0.12±0.01 Bq l −1 and 0.10±0.02 Bq l −1 in 15 tap water samples, respectively. The mean 222 Rn concentration of 1.40±0.03 Bq l −1 in the 20 groundwater samples of this study is also lower than the WHO advised level of 100 Bq l −1 . Fifty percent of groundwater samples analysed have 226 Ra levels in excess of the USEPA recommended maximum contaminant level of 0.185 Bq l −1 . The occurrence of elevated concentrations of 226 Ra in groundwater samples was explained by pH and alkaline conditions. [ABSTRACT FROM AUTHOR]