The glycosphingolipids isolated from spermatozoa of a fresh-water bivalve, Hyriopsis schlegelii, have a unique structure containing one or two mannosyl residues. We synthesized the trisaccharide derivatives which constitute the partial structure of lipid I and/or II. Condensation of 3, 4, 6-tri-O-acetyl-2-deoxy-2-phthalimido-β-D-glucopyranosyl bromide with methyl 3-O-benzyl-4, 6-O-benzylidene-α-D-mannopyranoside in the presence of mercuric cyanide gave methyl 3-O-benzyl-4, 6-O-benzylidene-2-O-(3, 4, 6-tri-O-acetyl-2-deoxy-2-phthalimido-β-D-glucopyranosyl)-α-D-mannopyranoside in 80% yield. Condensation of the disaccharide bromide with the appropriate C-3 hydroxyl free mannose derivatives afforded the corresponding GlcNAcβ1→2Manα1→3Man derivatives (11 and 12).