Terrestrial isopods store cuticular calcium in large sternal deposits composed of an amorphous CaCO3 compound. A large part of the deposits consists of numerous small spherules that increase the exposed surface to facilitate resorption of CaCO3 during cuticle mineralization. It is not known how these spherules are formed and how they are dissolved. This paper presents for the first time an analysis of ultrastructural changes occurring in the sternal CaCO3 deposits of a terrestrial isopod during their formation and degradation. Our results indicate that formation of the spherules takes place in a specialized aggregation zone, in which 10- to 30-nm-thick granules form agglomerations that then increase in size to form spherules that reveal a concentric growth pattern. Degradation of the deposits occurs in a manner that exposes a maximum of surface area on all levels of their structural organization. (C) 2003 Elsevier Science (USA). All rights reserved.