The mussel byssus is a unique holdfast structure employed by marine mussels to colonize diverse substrates. The byssus consists of extracellular threads with mainly proteinaceous components. Individual threads reveal high tensile strength at their distal end and high elasticity in their proximal portion. Our studies show that proteins of the distal part are oriented along the thread axis and are well-ordered with a high beta-structural content. In contrast, proteins of the proximal part are less ordered and are not as well-oriented with primarily alpha-helical structure. The detected differences in the structural features of the proteins along a byssus thread are likely an important basis for its gradual mechanical properties.