Abstract The deuterium abundance of natural water of various sources has been determined by the use of a deuterium mass spectrometer. The absolute deuterium contents of the reference sample nos. 1 and 1a of the U.S. National Bureau of Standards were found to be 0·0149 and 0·0128 mole % D2O, respectively, by the comparison of the samples with the standard samples of known deuterium content. Relative enrichment of no. la to no. 1 (R1a−R1)/R1×100 was 14·13 ± 0·23, which agrees perfectly to the value reported by Craig . The variation of deuterium contents of tap waters of Tokyo and Osaka during last 2 years were within ± 1 per cent and any seasonal regularities could not be detected. Marine waters of the West Pacific have almost the same deuterium content as those of the Pacific coast of the United States. Marine waters at various depths of the Antarctic showed relatively large variation of deuterium content (from +0·84 to −1·16 per cent on the SMOW-scale), whereas the samples of Black Current of the Pacific showed little variation. It was found that the deuterium content of spring water suggests the origin of springs in some cases.