The elastic constants of natural single-crystal aragonite (CaCO3) have been measured by Brillouin spectroscopy at ambient conditions. The elastic constants C-11, C-22, C-33, C-44, C-55, C-66, C-12, C-13 and C-23 are 171.1 1.0, 110.1 0.9, 98.4 1.2, 39.3 0.