Byssal threads of the common mussel Mytilus edulis contain collagenous molecules from which two pepsin-resistant fragments have been isolated and characterized. These show a complementary distribution along the length of the thread, such that one predominates distally (Col-D) and the other proximally (Col-P). Both fragments contain three identical alpha-like chains with molecular masses of 50 kDa (Col-P) and 60 kDa (Col-D) and have typically collagenous amino acid compositions; for example, 35% glycine and almost 20% proline plus 4-trans-hydroxyproline. Hydroxylysine and 3-hydroxyproline were absent. Col-P sequences are also typical of collagen in consisting of tandem repeats of the triplet Gly-X-Y in which X and Y generally represent any amino acid. When proline occurs, it is hydroxylated to 4-trans-hydroxyproline only in the Y position. Seven instances where X is glycine have been detected in Col-P. Specific polyclonal anti-Col antibodies were used to isolate the precursors of Col-P and Col-D from the mussel foot. PreCol-P has a molecular mass of 95 kDa and contains 36% glycine but a lower imino acid content (13%). It has a complementary distribution with another precursor (preCol-D, 97 kDa) along the length of the foot. The two precursor compositions suggest resilin-like and silk-fibroin-like structures, respectively, in the noncollagenous domains of preCol-P and preCol-D. Immunogold labelling studies indicate that Col-P is associated with the coiled fibers of the inner core in the proximal portion of the thread, whereas Col-D is localized to the straight fiber bundles of the distal thread as well as to the outer core of the proximal thread.