- Mobilization of calcium during the molt cycle from the cuticle to transient calcium deposits is widely spread in crustaceans. The dynamics of calcium transport to transient calcium deposits called gastroliths and to the cu- ticle over the course of the molt cycle were studied in the crayfish Cherax quadricarinatus. In this species, calcium was deposited in the gastroliths during premolt and trans- ported back to the cuticle during postmolt, shown by digital X-ray radiograph analysis. The predominant mineral in the crayfish is amorphous calcium carbonate embedded in an organic matrix composed mainly of chitin. Scanning elec- tron micrographs of the cuticle during premolt showed that the endocuticle and parts of the exocuticle were the source of most of the labile calcium, while the epicuticle did not undergo degradation and remained mineralized throughout the molt cycle. The gastroliths are made of concentric layers of amorphous calcium carbonate intercalated between chiti- nous lamella. Measurements of pH and calcium levels dur- ing gastrolith deposition showed that calcium concentra- tions in the gastroliths, stomach, and muscle were about the same (10 to 11 mmol l 1 ). On the other hand, pH varied greatly, from 8.7 0.15 in the gastrolith cavity through 7.6 0.2 in muscle to 6.9 0.5 in the stomach.