We analyzed ion composition and volume of the hemolymph of Ligia pallasii in four different stages of the molt cycle using capillary electrophoresis and 3H-inulin. The main ions in the hemolymph were Na+, K+, Mg2+ , Ca2+, and Cl-. The Ca2+ concentration increased significantly during the molt by 47% from intermolt to intramolt and by 37% from intermolt to postmolt, probably due to resorption of Ca2+ from the cuticle and sternal CaCO3 deposits. The K+ concentration increased significantly by 20% during molt. The hemolymph volume normalized to the dry mass of the animals decreased by 36% from intermolt to late premolt. This was due to a reduction in the hemolymph volume and to an increase in dry mass of the animals during premolt. A sudden increase in the hemolymph volume occurring between late premolt and intramolt served to expand the cuticle. Since the Na+, K+, Mg2+, and Cl- concentrations did not change significantly from late premolt to intramolt, the increase in hemolymph volume suggests an uptake of seawater rather than freshwater.