ABSTRACTCalcification of the new exoskeleton is the culmination of a successful molt in crustaceans. This study showed that inMacrobrachium rosenbergiicalcium present in the intermolt exoskeleton is moved internally as the molt approaches. Cyclic variations in the calcium concentrations of hemolymph, hepatopancreas and exoskeleton occur between successive intermolt periods. During premolt, hemolymph calcium concentration increases from an intermolt level of 7.67 to 14.8 mmole Ca+2liter‐1while in the hepatopancreas the change is from 0.77 to 1.22 mmole Ca+2·g dry wt‐1. An increase in calcium concentration of the exoskeleton appears to occur over this period (from intermolt 2.92 to 6.68 mmole Ca+2· g dry wt‐1). Thus, there is a mobilization of calcium to internal storage sites during premolt. Both the hemolymph and the hepatopancreas store calcium during premolt. Hemolymph and hepatopancreas stores are exhausted b the mid‐postmolt phase, falling to a minimum of 4.61 mmole Ca+2liter‐1and 0.31 mmole Ca+2·g dry wt‐1, respectively, while total calcium increases, from 0.56 to 1.39 mmole ·g dry wt‐1. The molting process causes a total calcium deficit that must be repaid from internally compartmentalized stores or from some external calcium source. Although internal sources of calcium are important for postmolt calcification inM. rosenbergii, our study indicates that additional uptake of calcium from freshwater may be necessary for complete calcification o