The assembly of myosin and paramyosin into filaments in muscle has been shown to depend in part on the interactions of regular periodic patches of charge on the surface of the rod regions of these alpha-helical coiled-coil proteins. It has also been known for some time that a relatively small region near the C-terminus of both molecules is critical for both solubility and assembly, This domain appears to function as a modulator of assembly in both proteins. Recently, a specific 29-residue region in the C-terminus of human fast muscle myosin rod has been shown to be essential for filament formation, and this sequence has been shown to be present in other vertebrate and invertebrate myosins. We show here that paramyosin also displays this specific conserved domain. Moreover we have found that this domain is part of a longer distinctive region in both paramyosin and myosin: this region lacks the periodic variation in charge found in the rest of both coiled coils, has a unique charge profile, a relatively neutral total charge, and a high proportion of large apolar residues in surface positions. These results may be useful in designing site-directed mutagenesis studies to identify the target regions on neighboring molecules which interact with this C-terminal domain and so establish the mechanism of its function.