Many marine animals such as mussels and barnacles are able to anchor themselves to surfaces through the use of proteinaceous adhesives, which have the unique ability to form strong bonds in a wet environment. With eventual goal of developing wet-surface adhesives, we have carried out gene sequencing studies, using three species of mussel, in order to learn what structural characteristics give these proteins their special properties. Two classes of protein are revealed. Both are characterized by containing about 20% each of lysine and of tyrosine (or DOPA). However, the proteins from Mytilus edulis and Mytilus californianus consist primarily of tandem repeats of variations of the proline-rich decapeptide sequence, AKPSYPPTYK, while the protein from Geukensia demissa is rich in glycine and glutamine and is comprised of 11- and 13-residue variants, e.g., GKPTITYDAGYK, GQQKQTGYDTGYK and GGVQKTGYSAGYK. We propose that the repeat domains have definite folded structures, if not in solution, at least in the condensed (crosslinked) state that gives them their distinctive properties. In this respect, the mussel proteins may resemble collagen, which has both secondary structure and adhesive properties.