Large, robust Strombus gigas lived in a variety of habitats in the Ironshore Lagoon that existed on Grand Cayman during the Late Sangamon highstand, approximately 125,000 years ago. Some well-preserved shells are still entirely aragonitic, whereas others have been transformed completely to calcite. Permeability of the host rock and sponge-boring intensity were the most important controls on the extent of aragonite-to-calcite inversion. Sponge borings played a critical role in the inversion process because they provided conduits through which the reactive diagenetic fluids could penetrate the dense gastropod skeleton. Each boring acted as a nucleus from which inversion fronts could spread into the shell. As a result, the radial fabric of the neomorphic calcite crystals around the sponge borings contrasts sharply with the mosaic of equant calcite that formed in areas devoid of sponge borings. It seems that bioerosion is, at least on a local scale, as important as other factors in controlling the meteoric alteration of carbonate sediment. The delta 18 O values of neomorphic calcite (-7.1 per thousand to -4.9 per thousand ), approximately 3 per thousand more depleted in 18 O than the original aragonite, confirm the role of meteoric water in the aragonite-to-calcite transformation. delta 13 C values of the diagenetic calcite (+0.2 per thousand to +4.5 per thousand ) are similar to those in the original aragonite, suggesting that the former was inherited from the latter.