Crustaceans must replace their old exoskeleton with a new, larger one in order to grow and differentiate. During that process a new (pre-exuvial) cuticle is deposited beneath the old cuticle. Since breakdown products from the old cuticle have been reported to be removed from the old cuticle and transported across the new cuticle and hypodermis into the hemolymph, the preexuvial cuticle is presumed to be permeable. However, it has also been reported that intermolt cuticle is a highly impermeable structure that prevents loss of water and ions from the cuticle to the external environment. This study was designed to determine the timing of any change in the permeability of the cuticle of the blue crab, Callinectes sapidus, from the period just preceding ecdysis through the early postmolt period. To test for any changes in permeability pilot studies were performed on pieces of dorsobranchial cuticle from late premolt (D₄) and early postmolt crabs (15 min, 2 h, 12 h and 18 h post-ecdysis) using ³H₂O and an Ussing chamber. Additional permeability studies were done monitoring the movement of p-nitrophenol (pNP) across the cuticle using a more restricted post-exuvial time course (0, 5, 10, 15 and 30 min. post-ecdysis) as compared to premolt cuticle. Results from these studies as well as wash-out studies using tissues pre-loaded with pNP and staining of 1 h post-molt cuticle with OsO₄ suggest that the epicuticle undergoes a transition at or soon after ecdysis that renders it impermeable to water and small molecules. While the precise mechanism for this transition is unknown, it is clear that the resultant alteration in epicuticle permeability during early post-ecdysis allows for the formation of a barrier that prevents osmotic and ionic exchange, allows for the generation of significant hydrostatic pressure, and provides a suitable microenvironment for calcification.