We report that extracts ofSchistosoma mansonicontain a GDPFuc:Galβ1–4GlcNAc (Fuc to GlcNAc) α1–3 fucosyltransferase (α1,3FT) capable of synthesizing the antigenic determinant known as Lewis x (Le x , Galβ1–4[Fucα1–3]GlcNAcβ1–R). When the acceptor lacto-N-neotetraose (LNnT, Galβ1–4GlcNAcβ1–3Galβ1–4Glc) was incubated with extracts ofS. mansoniin the presence of GDPFuc and Mn 2+ , Fuc was transferred to generate the pentasaccharide lacto-N-fucopentaose III (LNFPIII, Galβ1–4[Fucα1–3]GlcNAcβ1–3Galβ1–4Glc). The enzyme did not transfer efficiently to the isomeric oligosaccharide lacto-N-tetraose (LNT, Galβ1–3GlcNAcβ1–3Galβ1–4Glc). The activity of the schistosome α1,3FT toward LNnT was dependent upon time, protein, and GDPFuc. Interestingly, the schistosome α1,3FT was also able to transfer Fuc to a sialic acid-containing trisaccharide NeuAcα2–3Galβ1–4GlcNAc to produce the tetrasaccharide sialyl Lewis x (2,3 sLe x , NeuAcα2–3Galβ1–4[Fuc1–3]GlcNAc), although the rate of reaction with the sialylated acceptor was <5% of the rate obtained toward nonsialylated acceptor. The schistosome α1,3FT was relatively resistant to inhibition byN-ethylmaleimide. The enzymatic properties of the schistosome α1,3FT resemble those of the human myeloid fucosyltransferase FTIV and not those of other known human fucosyltransferases.