Many marine bivalve molluscs produce byssal threads for attachment to solid substrata. Small (less than 10 mm) consecutive sections of the byssal threads of Mytilus edulis, M. californianus, Geukensia demissa, Atrina vexillum, and A. rigida were analyzed by amino acid analysis to determine if chemical composition remains constant as a function of location in thread segments. Nonlinear longitudinal protein gradients, probably involving collagen and an elastic protein, were found in the Mytilus species. In these, collagen peaks in the distal third of the thread. In Geukensia and the Atrina species, although the two differed greatly in composition, there is a clear nonvariability in composition of the thread within each species as a function of location in the thread. The adhesive plaque at the tip of the thread of all species examined differs substantially in composition from the remainder of the thread. Protein gradients in the threads of some bivalves may reflect specific adaptations evolved to respond to exposed habitats in high-energy environments.