We have conducted the first study on the effect of ergocalciferol (vitamin D-2) on the polymorphism of calcium carbonate. CaCO3 particles are synthesised from aqueous solutions of sodium carbonate and calcium chloride, in the presence of vitamin D-2 with different concentrations, at temperatures ranging from 25 to 80 degrees C. The effect of the additive on the polymorphism and the morphology of the CaCO3 crystallites are characterised using X-ray quantitative phase analysis, scanning electron microcopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Our results show that the presence of vitamin D-2 has an impact on the morphology and composition of the CaCO3 crystallites. Vitamin D-2 was demonstrated to promote the formation of aragonite a 50 degrees C with a volume fraction that can reach 83%. Such effect can be attributed to the formation of D-2-Ca+2 ion solution complexes and their adsorption on the surfaces of the CaCO3 crystallites.