Mineralized biological concretions have attracted increasing interest because of their outstanding properties. The mineralized concretion of terrestrial isopods is an excellent model for acellular natural composite material. Before the molt terrestrial isopods resorb calcium from the posterior cuticle and store it in concretion within the cranial (head) and caudal (tail) ventral segments. This paper present for the first time an analysis of ultrastructural changes occurring in the caudal ventral segmental (CaVS) concretion of a terrestrial isopod Porcellius chilensis during their formation and degradation. The CaVS concretion of the woodlice Porcellius chilensis was analyzed with respect to their content of inorganic material. It was found that the concretion consists of amorphous calcium carbonate (ACC), and amorphous calcium phosphate (ACP), besides small amounts of water and an organic matrix. The CaVS concretion consists of structurally distinct stratum due to inhomogeneous solubility of ACC within the organic matrix that consists of calcareous knob with reticules elements. The organic matrix plays a role in the structural organization of the concretion and in the stabilization of ACC, which is unstable in vitro. We present an analysis of the distribution of minerals, elements, and organic matrix with in the CaVS concretion by using SEM, XRD, IR and EDS. The decalcification experiments exactly imitated the natural demineralization of the CaVS concretion of the Porcellius chilensis and it is thought that an inhomogeneous solubility of ACC and ACP within the CaVS concretion probably caused by variations in the stabilizing properties of matrix components.