Carbon isotope ratios were determined for carbonates from the banded iron-formation of the Dales Gorge Member of the Brockman Iron Formation and from the Wittenoom and Duck Creek Dolomite Formations of the Hamersley Range area of Western Australia. The iron-formation carbonates had δC13 values of -9 to -11 %., while the dolomitic limestones had δC13 values of +2 to -2 %. It is concluded that the iron formation was precipitated in a basin isolated from the ocean, but probably in close proximity to it and that organic activity may have played a significant role in the genesis of the iron-formation. The dolomitic limestones were deposited during periods when the basin was transgressed by the ocean. A consideration of the available data on Precambrian marine carbonates leads to the conclusion that the oceanic reservoir of carbon has remained nearly constant for almost 3 × 10 9 years. The argument previously advanced for Phanerozoic samples, that this constancy implies a constant amount of atmospheric oxygen is reconsidered, and is found to rest on questionable assumptions.