Posterior isolated gills of Neohelice (Chasmagnathus) granulata were symmetrically perfused with haemolymph-like saline of varying [HCO₃^-] and pH. Elevating [HCO₃^-] in the saline from 2.50 to 12.50 mmol l^-1 (pH 7.75 in both cases) induced a significant increase in transepithelial potential difference (V_te), a measure of ion transport. The elevation in [HCO₃^-] also induced a switch from acid secretion (-43.7 ± 22.5 µequiv kg^-1 h^-1) in controls to base secretion (84.7 ± 14.4 µequiv kg^-1 h^-1). The HCO₃^--induced V_teincrease was inhibited by basolateral acetazolamide (200 µmol l^-1), amiloride (1 mmol l^-1) and ouabain (5 mmol l^-1), but not by bafilomycin (100 nmol l^-1). The V_te response to HCO₃^- did not take place in Cl^--free conditions; however, it was unaffected by apical SITS (2 mmol l^-1) or DIDS (1 mmol l^-1). A decrease in pH from 7.75 to 7.45 pH units also induced a significant increase in V_te, which was matched by a net increase in acid secretion of 67.8 ± 18.4 µequiv kg^-1 h^-1. This stimulation was sensitive to basolateral acetazolamide, bafilomycin, DIDS and Na⁺-free conditions, but it still took place in Cl^--free saline. Therefore, the cellular response to low pH is different from the HCO₃^--stimulated. We also report V-H⁺-ATPase- and Na⁺/K⁺-ATPase-like immunoreactivity in gill sections for the first time in this crab. Our results suggest that carbonic anhydrase (CA), basolateral Na⁺/H⁺ exchangers and Na⁺/K⁺-ATPase and apical anion exchangers participate in the HCO₃^--stimulated response, while CA, apical V-H⁺-ATPase and basolateral HCO₃^--dependent co-transporters mediate the response to low pH.