The respiratory and acid-base responses of the supratidal crab Leptograpsus variegatus to immersion in either 100% or 50% seawater were measured over a 3-wk period. Leptograpsus maintained haemolymph PO2 and CO2 during immersion, owing partly to elevated cardiac output and without recourse to hyperventilation. Both haemolymph PCO2 and CCO2 were reduced following 3 wk of immersion, and L-lactate remained low, but nonetheless Leptograpsus exhibited a marked metabolic acidosis. Specific responses to 50% seawater exposure were observed in Leptograpsus when compared with control crabs immersed in 100% seawater. Improved O2 delivery concomitant with elevated perfusion and ventilation maintained haemolymph oxygenation. In hyposaline-exposed Leptograpsus, both PCO2 and CCO2 were reduced after 24 h, while pH was elevated after 15 min and remained so during the 3-wk immersion. The marked acidosis exhibited in Leptograpsus exposed to 100% seawater was partially reduced in those in 50% seawater, presumably because of net acid excretion associated with elevated Na+ uptake. Immersion of Leptograpsus reduced Hc-O2 affinity to a greater extent in hyposaline-immersed individuals, while the cooperativity of O2 binding by Leptograpsus Hc decreased at low pH in both salinities. When immersed, the respiratory status of Leptograpsus resembles that of water-breathing crabs. Exposure to 50% seawater causes a twofold increase in energetic cost but does not perturb the immersed condition. The metabolic acidosis induced by chronic immersion is partially compensated at the lower salinity. Leptograpsus thus has the ability to take advantage of habitats ranging from supratidal through brackish to fully marine.