The oligosaccharide structures of the structural subunit HtH1 of Haliotis tuberculata hemocyanin (HtH) were studied by mass spectral sequence analysis of the glycans. The proposed structures are based on MALDI-TOF-MS data before and after treatment with the specific exoglycosidases β1-3,4,6-galactosidase and α1-6(>2,3,4) fucosidase followed by sequence analysis via electrospray ionization MS/MS-spectra. In total, 15 glycans were identified as a highly heterogeneous group of structures. As in most molluscan hemocyanins, the glycans of HtH1 contain a terminal MeHex, but more interestingly, a novel structural motif was observed: MeHex[Fuc(α1-3)-]GlcNAc, including thus MeHex and (α1-3)-Fuc residues being linked to an internal GlcNAc residue. While the functional unit (FU) c (HtH1-c) is completely lacking any potential glycosylation site, FU-h possesses a second exposed sugar attachment site between beta-strands 8 and 9 within the beta sandwich domain compared to the other FUs. The glycosylation pattern/sites show a high degree of conservation. In FU-h two prominent potential glycosylation sites can be detected. The finding that HtH1 is not able to form multidecameric structures in vivo could be explained by the presence of the exposed glycan on the surface of FU-h.