Oniscidean isopods brood the eggs in a fluid-filled marsupium, and embryos undergo prolonged lecithotrophic development. After sloughing of the vitelline membrane (second embryonic molt), the mancas remain in the marsupium for several more days before emerging as free-living juveniles. Calcium is required for cuticle mineralization in crustaceans, and the time-course of net calcium uptake during these early developmental stages is thus of interest. We studied calcium uptake during embryogenesis and manca development in Armadillidium vulgare using Ca microelectrodes and atomic absorption spectrophotometry. Total Ca increased approximately 17-fold during egg development, from 0.33 ± 0.12 nmol in Early Stage 1 eggs to 5.58 ± 2.73 nmol in Late Stage 2 eggs. However, the major period of uptake occurred after the second embryonic molt, with total Ca increasing rapidly to 197 ± 19.5 nmol per animal in Day 4 mancas; this represents a 35-fold increase from Late Stage 2 eggs. Mancas take up amaranth when cultured in vitro, showing that they imbibe fluid and probably acquire Ca in this way. Eggs reared in vitro in isopod saline show greatly reduced Ca accumulation, indicating that marsupial development provides other sources of Ca and/or metabolic substrates essential for normal uptake. Periembryonic fluid Ca concentrations measured in Late Stage 2 eggs (following blastokinesis) were 22.1 ± 3.5 mmol L⁻¹, almost double the marsupial fluid levels. Stage 2 eggs reared in vitro in low-Ca (1 mmol L⁻¹) saline show a similar capacity to up-regulate periembryonic Ca concentrations, and the trans-vitelline membrane potential (TVMP) of these eggs lies far from the calcium equilibrium potential (E_Ca). Inward calcium transport may occur across the dorsal organ; the same eggs show prolonged loss of Ca when the dorsal organ degenerates following blastokinesis.