Abstract Bacterial decomposition in the laboratory of butterfish, smelts, and Mya clams, and Venus clams in the presence of CaCO 3, over periods ranging from 65 to 250 days, results in a slow rise in the pH of sea water in contact with the decaying organisms. The rise in pH is due to the formation of ammonia and other nitrogenous bases from the breakdown of proteins and other nitrogen-containing biochemical compounds. In the case of butterfish and smelts the rise in pH brings about the precipitation of dissolved calcium as calcium fatty acid salts or soaps in the carbon range 14C- 18C. No CaCO 3 is formed. In the case of clams dissolved calcium is precipitated during pH rise only where there is extensive bacterial sulfate reduction; ammonia generation in itself does not bring about precipitation. Large amounts of acids other than carbonic acid are generated during decomposition of Venus clams. In the absence of CaCO 3 this results in a very low pH which does not subsequently rise.