Laboratory studies were undertaken to discover mechanisms which might explain the origin of calcareous ooliths in nature. Calcium carbonate was precipitated from sea water by evaporation, by addition of precipitating agents and by the growth of sulfate-reducing bacteria. The effect of the major ions in sea water on the form of the calcium carbonate precipitate was also studied. These experiments showed that calcareous ooliths can be formed by chemical precipitation from sea water and by the growth of sulfate-reducing bacteria in sea water. The presence of magnesium ion in sea water causes calcium carbonate to precipitate as aragonite rather than calcite. An environment in which fresh waters high in carbonate ion flow into a shallow sea would reproduce the laboratory conditions for chemical precipitation of calcareous ooliths. Such a mechanism does not appear to be responsible for any large present day deposits. Environments where sulfate-reducing bacteria might be important in the origin of calcareous ooliths should be characterized by deficiency of oxygen, increased pH and presence of hydrogen sulfide or sulfide salts. Such environments might be highly localized.