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We have performed precise structural measurements on five different calcitic seashells by high-resolution X-ray powder diffraction on a synchrotron beam line and by laboratory single crystal X-ray diffraction. The unit cell parameters a and c of biogenic calcite were found to be systematically larger than those measured in the non-biogenic calcite. The maximum lattice distortion (about 2·10⁻³) was detected along the c-axis. Under heat treatment above 200°C, a pronounced lattice relaxation was observed, which allowed us to conclude that anisotropic lattice swelling in biogenic calcite is induced by organic macromolecules incorporated within the single crystal calcitic prisms during biomineralization. This conclusion is supported by the results of crystallization experiments in the presence of specific protein extracted from one of the shells.