The effect of long-term estivation on the abundance of calcium granules in the lung tissue of the apple snail Pomacea maculata Perry was studied. Electron-dispersive spectrophotometric analysis of the shell and the granular inclusions in the lung show that both structures are composed of calcium carbonate. The layers forming the shell are similar to those found in the shells of other gastropods. Calcium granules are extremely abundant in the tissues of the lung of P. maculata. The spherical granules consist of concentric layers of material inside calcium cells. There are three distinct populations of hemocytes in P. maculata hemolymph, two cell types with granulocytic morphology and one agranular type. Selective staining shows that hemocytes contain large amounts of calcium, implicating these cells in the transport of calcium carbonate from storage sites to other tissues. The pCO₂ of the hemolymph increases in estivating gastropods, and it has been suggested that stores of solid calcium carbonate are mobilized to buffer the resulting acidity. Results of this study show that a large decrease in the abundance of calcium carbonate granules in the lung tissue occurs during estivation. In addition, there is evidence that a calcium binding protein is present in the hemocytes.