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Schistosoma mansoni masks its surface with adsorbed host proteins including erythrocyte antigens, immunoglobulins, major histocompatibility complex class I, and ␤ 2 -microglobulin (␤ 2 m), presumably as a means of avoiding host immune responses. How this is accomplished has not been explained. To identify surface receptors for host proteins, we biotinylated the tegument of live S. mansoni adults and mechanically transformed schistosomula and then removed the parasite surface with detergent. Incubation of biotinylated schistosome surface extracts with human immunoglobulin G (IgG) Fc-Sepharose resulted in purification of a 97-kDa protein that was subsequently identified as paramyosin (Pmy), using antiserum specific for recombinant Pmy. Fc also bound recombinant S. mansoni Pmy and native S. japonicum Pmy. Antiserum to Pmy decreased the binding of Pmy to Fc-Sepharose, and no proteins bound after removal of Pmy from extracts. Fluoresceinated human Fc bound to the surface, vestigial penetration glands, and nascent oral cavity of mechanically transformed schistosomula, and rabbit anti-Pmy Fab fragments ablated the binding of Fc to the schistosome surface. Pmy coprecipitated with host IgG from parasite surface extracts, indicating that complexes formed on the parasite surface as well as in vitro. Binding of Pmy to Fc was not inhibited by soluble protein A, suggesting that Pmy does not bind to the region between the CH2 and CH3 domains used by many other Fc-binding proteins. ␤ 2 m did not bind to the schistosome Fc receptor (Pmy), a finding that contradicts reports from earlier workers but did bind to a heteromultimer of labeled schistosomula surface proteins. This is the first report of the molecular identity of a schistosome Fc receptor; moreover it demonstrates an additional aspect of the unusual and multifunctional properties of Pmy from schistosomes and other parasitic flatworms.Schistosomes establish chronic infections in their hosts despite the presence of specific cellular and humoral immune responses. One of the most striking immunoevasive mechanisms displayed by these parasites is the acquisition of host products onto the tegument of the schistosome to mask its foreign status. Host molecules adsorbed onto the surface include immunoglobulins (20,40,44), major histocompatibility complex products (3, 36), ␤ 2 -microglobulin (␤ 2 m) (40), complement components (35, 38), ␣ 2 -macroglobulin (7, 19), C3 decay-accelerating factor (14), and glycolipids in the form of A, B, H, and Lewis blood group antigens (12).More than 20 years ago, Schistosoma mansoni schistosomula were shown to selectively bind the Fc but not the Fab fragment of immunoglobulin G (IgG) (40). This was demonstrated by the adhesion of rosetted sheep erythrocytes to transformed schistosomula of S. mansoni in vitro and inhibition of adhesion by free Fc fragments. IgG from rat, mouse, and rabbit also inhibited rosette formation. Whereas most of the host molecules listed above are adsorbed by schistosomula, receptors for Fc and complement C3 have also been ...