Calcium carbonate precipitation by bacterial activity was noted in seawater collected from offshore Bermuda as well as from Halifax Harbor, provided that a minimum of 70 mg/liter of organic matter was added in addition to trimethylamine‐N‐oxide (TMAO) or sodium nitrate (final minimum concentrations, 1.33 and 0.17 g/liter, respectively). Observations made during these investigations indicated that the pH was the most important factor in the initiation of carbonate precipitation; the addition of calcium acetate, sodium carbonate, calcium sulfate, or ammonium carbonate did not result in carbonate precipitation. Precipitation was noted in cultures only when the pH exceeded 8.3. Precipitation occurred in the absence of bacteria when the culture was filtered after the pH increased to above 8.3 (2 days after nutrient amendment). Filter‐sterilized seawater, adjusted to pH 8.4 or above with NH4OH, also precipitated carbonates. A study of 53 random isolates from Bermuda waters and sediments revealed that only 20% of the isolates were able to precipitate carbonates. In most cases, the calcium carbonate precipitating isolates raised the pH above 9.0. No isolates from Bermuda or Halifax Harbor have yet been discovered that are able to precipitate calcium carbonates at in situ pH values.