Adult parasites of Schistosoma mansoni reside within vertebrate mesenteric veins where they consume immense quantities of host glucose after transporting the sugar through their surface syncytium or tegument. Previously we obtained cDNA clones encoding two functional facilitated diffusion glucose transporter proteins expressed by S. mansoni adult worms (Skelly et al. 1994). Antibodies specific for one transporter (SGTP1) have been generated against an extrafacial and an internal domain of the protein and used to localize the protein by light and electron microscopy. By light microscopy both antibodies stain a linear structure approximately 1-5 microns from the surface of the tegument of adult male and female schistosomes. Electron microscopic examination of frozen thin sections show binding of the antibodies to membranes in the base of the tegument and not to the membranes covering the outer surface or their invaginations. Analysis of the gold distribution suggests that the extrafacial domain is disposed toward the interstitial space beneath the tegument and the internal domain faces the syncytial plasm. The localization suggests that SGTP1 may function to transport free glucose from within the tegument and into the interstitial fluids that bathe the internal organs of these parasites.