Abstract The distribution of aragonite in the skeletal parts of living organisms is reviewed, and its distribution in fossils is described on the basis of several hundred new determinations. Aragonitic fossils are extensively preserved in Tertiary sediments, and are common in Mesozoic rocks, particularly where the enclosing lithology is argillaceous. No aragonite was found by the authors in fossils from Palaeozoic rocks. The most important requirement for the preservation of aragonite is the presence of reducing conditions. It is suggested that the preservation of aragonite is due to the protective effect of organic skeletal matrix, which in turn requires reducing conditions for its preservation. The protective action of the organic matrix is attributed to the formation of a hydrophobic monomolecular layer on the crystal surface, composed of aminoacids derived from the breakdown of skeletal matrix proteins.