The anterior aorta and major arteries of the land pulmonate snail Anguispira alternata have large calcium deposits in their walls. These deposits occur inside spherule cells, which line the walls of these vessels. The calcium occurs as amorphous calcium carbonate, in the form of intracellular spherules having alternating layers of organic and inorganic material. The spherule cells appear to be degenerating connective tissue cells; they are characterized by large numbers of spherules and by a cytoplasm which is totally empty except for a nucleus, scattered glycogen particles and a few membrane remnants. The injection of 45Ca into the foot of the snail results in rapid incorporation of this calcium into the spherule cells. Although calcium-containing spherules are now known from a wide variety of invertebrate tissues, they have not been previously recorded from arterial walls. The physiologic significance of these deposits is not known.