Ultrahistochemical evidence for quercitin-sensitive ATPase activity, believed to represent Ca-ATPase activity, in the epidermal tissue underlying the exoskeleton (=hypodermis) of the marine crab, Callinectes sapidus, is presented for different stages of the moult cycle. At premoult stage D 2 , activity in the epidermis was localized along the basolateral membrane below the apical junctions, but at D 3 , activity was only apparent in tissue associated with the carapace suture lines. No evidence of Ca-ATPase activity was found immediately after the moult (A 1 ), but by A 2 , calcification was occurring in the cuticle, and activity was present along the apical membranes and microvilli. At B 2 similar activity was seen, and the apical cytoplasm contained small vacuoles showing activity on their bounding membranes. No activity was seen in intermoult (C 4 ). These data were interpreted to mean that calcium entered the hypodermal cells from the cuticle in D 2 and was pumped out into the intercellular spaces by the activity of Ca-ATPase. This movement decreased greatly as the moult approached (D 3 ) except in the vicinity of the suture lines where resorption of calcium is known to be more extensive. In postmoult stages, the enzyme activity, concentrated at the apical surface of the epidermal cells, was consistent with the net outward transport of calcium toward the cuticle.