This paper reports the adaptation of a branchial perfusion technique to the gills of the small hyper-hyporegulating crab, Pachygrapsus marmoratus. The physiological quality of the preparation has been established and preliminary measurements of the transepithelial potential difference (PD) and sodium fluxes were reported. A clear-cut distinction has to be made between anterior and posterior gills. With symmetrical bathing conditions (same saline on both sides of the epithelium), a significant transepithelial PD was measured only in posterior gills isolated from crabs acclimated to dilute sea water. This hemolymph-side negative PD is shown to require biological energy and to be sensitive to ouabain. Na+ influx was larger than efflux, indicating the occurrence of a net inward of Na+. The active nature of the Na+ influx was established.