<jats:title>SUMMARY</jats:title><jats:p>We studied the transepithelial potential difference (TEPD) and 22Na flux across isolated perfused gills (anterior pair 5 and posterior pairs 6–8) of the crab Chasmagnathus granulatus acclimated to either hypo- or hyper-osmotic conditions.</jats:p><jats:p>The gills of crabs acclimated to low salinity, perfused and bathed with 10 ‰ saline solutions, produced the following TEPDs (hemolymph side with respect to bath side): 0.4±0.7, –10.2±1.6, –10.8±1.3 and –6.7±1.3 mV for gills 5, 6, 7 and 8, respectively. Gills 6, 7 and 8 did not differ significantly. Reducing the saline concentration of bath and perfusate from 30 ‰ to 20 ‰ or 10 ‰ increased significantly the TEPDs of these gills. TEPDs of gill 6 (representative of posterior gills) were reduced by 69±5 % and 60±5 % after perfusion with ouabain or BaCl2 (5 mmol l–1 each), respectively. The same gill showed a net ouabain-sensitive Na+ influx of 1150±290 μequiv g–1 h–1.</jats:p><jats:p>Gill 6 of crabs acclimated to high salinity produced TEPDs of –1.5±0.1 and –1.3±0.09 mV after perfusion with 30 ‰ or 40 ‰ salines, respectively. Perfusion with ouabain or BaCl2 reduced TEPDs by 76±7 % and 86±4 %, respectively. A net ouabain-sensitive Na+ efflux of 2282±337 μequiv g–1 h–1 was recorded in gill 6 perfused with 38 ‰ saline.</jats:p>