Recently, the novel C-glycosidic linkage of a hexopyranosyl residue to the indole ring of tryptophan residue 7 of human RNase U(s) was reported [Hofsteenge, J., Müller, D. R., de Beer, T., Löffler A., Richter, W. J., & Vliegenthart, J. F. G. (1994) Biochemistry 33, 13524-13530]. Identification of this monosaccharide is a prerequisite for studies of its biosynthesis and its biological relevance. Using vicinal proton-proton coupling constants and rotating-frame nuclear Overhauser enhancements, ewe demonstrate that the C-linked substituent is alpha-mannopyranose. Furthermore, the nuclear magnetic resonance (NMR) data indicate that the mannopyranose moiety in a glycopeptide derived from RNase U(s) adopts several conformations on the NMR time scale.