Calcium deposits of the first four anterior sternal plates of the terrestrial isopod Porcellio scaber must be quickly mobilized after ecdysis of the posterior half of the cuticula. The structural configuration of these deposits was examined by scanning electron microscopy, energy filtered transmission electron microscopy, and energy spectroscopic electron diffraction analysis. The deposits consist of an opaque region built by spherules of diameters varying from 0.3 to 1.7 mu m and a translucent layer glassy in appearance. Both parts are composed of an organic matrix and an amorphous calcium carbonate compound. The deposition of calcium carbonate in numerous small spherules and in an amorphous form probably facilitates its quick mobilization. (C) 1994 Academic Press, Inc.