TITIN is the largest polypeptide yet described (relative molecular mass ~3xl06 refs 1,2) and an abundant protein of striated muscle3. Its molecules are string-like4 and in vivo span from the M to Z-lines5,6. I-band regions of titin are thought to make elastic connections between the thick filament and the Z-line5-7, thereby forming a third type of sarcomere filament8,9. These would centre the A-band in the sarcomere and provide structural continuity in relaxed myofibrils10. The A-band region of titin seems to be bound to the thick filament, where it has been proposed to act as a 'molecular ruler' regulating filament length and assembly6. Here, we show that partial titin complementary DNAs encode a regular pattern of two types of 100-residue motif, each of which probably folds into a separate domain type. Such motifs are present in several evolutionarily divergent muscle proteins, all of which are likely to interact with myosin. One or both of the domain types is therefore likely to bind to myosin.