The precipitation of calcium carbonate (CaCO3) from saturated solutions prepared using calcium chloride and sodium hydrogen carbonate under the influence of ultrasound (20 kHz, intensity ranging from 1.5-18.5 W cm(-2)) has been studied. For short reaction times or treatment at low intensity, precipitates containing up to 90% vaterite were recovered; longer (> 30 min) times or higher intensities yielded up to 100% calcite. Changes in the crystal sizes were noted. The results are consistent with the literature model of CaCO3 crystallization by the initial formation of vaterite followed by a dissolution recrystallization process. Although a complete explanation cannot be offered, the results help to explain some of the apparently conflicting information previously published on this topic.