The effect of alginic acid, extracted from the Lessonia trabeculata calcareous marine algae, was studied in the electrocrystallization of calcium carbonate (CaCO3) on indium tin oxide (ITO) transparent electrodes. Scanning electron microscopy, X-ray diffraction-collected data and microscopic observations show that the presence of Mg2+ at the electrolyte induces the magnesium calcite and aragonite phase formation along with the changes in morphology and crystal orientation. Addition of alginic acid strongly inhibits the effect of Mg2+ on the crystallization, receiving in favor of the nucleation and growth of calcite phase of CaCO3. These results suggest that the alginic acid acts as an organic template in the electrocrystallization of CaCO3, controlling the crystallographic orientation of crystal growth, and as a consequence, the crystal morphology. (c) 2005 Elsevier B.V. All rights reserved.