The growth of calcium carbonate crystals within thermally evaporated aerosol OT (AOT) thin films is described. In situ crystal growth is accomplished by a process of Ca2+ ion entrapment in the AOT host followed by CO32- ion reaction. The CACO3 crystals exhibited almost 100% aragonite crystal structure and were in the form of bundles of aragonite needles. The fact that a similar ion exchange process in thermally evaporated stearic acid films resulted in growth of oriented calcite crystals underlines the role of the lipid matrix in determining the crystal structure of the nucleating phase. Contact angle measurements of the CaCO3-AOT films indicated that they were hydrophobic, thus pointing to the possible role of hydrophobic interaction between the AOT monolayer-covered CACO3 crystals in the assembly process.