The results of a study of the role of organic compounds in the formation of carlxmate crystals in marine biological systems are reported. In an increasing concentration of certain organic compounds which complex calcium ions, the proportion of aragonite decreases and that of calcite increases. In increasing concentrations of magnesium ions the proportion of aragonite increases and that of calcite and vaterite decreases. When the influence of organic compounds is greater or smaller than that of magnesium ions, only calcite or only aragonite is formed, respectively. Organic compounds forming a strong complex with calcium ions cause the formation of magnesium-rich calcite, and with an increase in temperature and the concentration of magnesium ions, the magnesium carbonate content of precipitated magnesian calcite increases. When the influence of organic compounds is almost equivalent to that of magnesium ions, in increasing or decreasing temperatures, the proportion of calcite decreases or increases, respectively, and the proportion of aragonite increases or decreases, respectively. The concentration of magnesium ions in the body fluids of marine calcareous organisms seems to differ little from that of other organisms, and seems to be similar to that of sea water. Only the presence of certain organic compounds brings about the formation of the carbonate crystals observed in marine biological systems. The very important role of organic matter in the formation of crystals found in skeletal carbonates is emphasized.