The Schistosoma mansoni protein, SPRM1lc, is a light chain member of a new family of heterodimeric amino acid permeases. These proteins require covalent association with a type II glycoprotein (like h4F2hc) for functional surface localization when expressed in Xenopus oocytes. We previously reported that, when co-expressed with h4F2hc, the transport properties of SPRM1lc resemble system y and y+ while its human homologue, E16, functions as an L-type permease. Here we extend the functional characterization of SPRM1lc in oocytes and show by competitor studies that its amino acid transport capacity is similar to that of whole adult schistosomes. We demonstrate by Northern and Western analysis that SPRM1lc is expressed within both larval and adult schistosomes. In all stages, SPRM1lc is associated into a high molecular weight complex that can be disrupted by reducing agents, consistent with the hypothesis that a significant fraction of the endogenous SPRM1lc is linked by a disulphide bond to an uncharacterized schistosome amino acid transporter heavy chain. Immunofluorescence localization detects SPRM1lc in miracidia, daughter sporocysts and adult worms. Confocal microscopy demonstrates that SPRM1lc is found in the apical membrane of the syncytial, double-lipid bilayer tegument which surrounds adult worms. Aqueous biotinylation studies on living worms show that SPRM1lc is exposed on the host-interactive surface of this tegumental membrane. Host exposed, functionally important surface proteins such as SPRM1lc could form the basis of an effective schistosomiasis vaccine. These studies are the first to describe a helminth amino acid transporter, and the first to characterize an invertebrate heterodimeric amino acid transporter.