Phosphate transport across plasma membranes has been described in a wide variety of organisms and cell types including gastrointestinal epithelia. Phosphate transport across apical membranes of vertebrate gastrointestinal epithelia requires sodium; whereas, its transport across the basolateral membrane requires antiport processes involving primarily chloride or bicarbonate. To decipher the phosphate transport mechanism in the foregut apical membrane of the mollusc, Aplysia californica, in vitro short-circuited Aplysia californica gut was used. Bidirectional transepithelial fluxes of both sodium and phosphate were measured to see whether there was interaction between the fluxes. The net mucosal-toserosal flux of Na was enhanced by the presence of phosphate and it was abolished by the presence of serosal ouabain. Similarly, the net mucosal-to-serosal flux of phosphate was dependent upon the presence of Na and was abolished by the presence of serosal ouabain. Theophylline, DIDS and bumetande, added to either side, had no effect on transepithelial difference or short-circuit current in the Aplysia gut bathed in a Na₂HPO₄ seawater medium. However, mucosal arsenate inhibited the net mucosal-to-serosal fluxes of both phosphate and Na and the arsenate-sensitive Na flux to that of phosphate was 2:1. These results suggest the presence of a Na-PO₄ symporter in the mucosal membrane of the Aplysia californica foregut absorptive cell.