AbstractPreparations of posterior gills isolated from postmoult shore crabs (Carcinus mediterraneus) that had been acclimated to dilute seawater were used for calcium‐transport studies. Gills were perfused and bathed with saline of identical composition, with 45Ca2+ ions added exclusively to the bathing solution or the perfusate for measurement of unidirectional influx or efflux, respectively. The unidirectional influx of calcium (5.57 ± 0.95 μmol h−1 per g gill fresh weight) significantly exceeded the efflux (3.15 ± 1.02 μmol h−1 per g gill fresh weight), indicating a net transgill influx of calcium.Calcium influxes were inhibited when La3+ ions were added apically (IC50 = 7 × 10−6 mol 1−1) or basolaterally (IC50 = 1.6 × 10−6 mol 1−1). In addition, basolaterally applied La3+ ions inhibited effluxes with an IC50 of 3.0 × 10−7 mol 1−1. Amiloride had no significant effect on the unidirectional fluxes of calcium. Influxes of calcium gave a sigmoidal curve when plotted against the external concentration of calcium in the bath. The Hill coefficient (2.66 ± 0.20) suggested that there are at least two or three Ca2+ carrier systems that are active during the influxes of calcium. Moreover, apically added La3+ ions produced a concentration‐dependent inhibition of calcium influx, which is discussed in terms of inhibition of apically located calcium channels. By contrast, the inhibitory action of La3+ ions at the basolateral gill surfaces is discussed in terms of an effect on ATP‐dependent calcium transport and the Na+/Ca2+ exchanger. © 1994 Wiley‐Liss, Inc.