Abstract Properties of the amino acid sequence of the nematode myosin rod region, deduced from cloned DNA, are analysed. The rod sequence of 1117 residues contains a regular region of 1094 residues, which has features typical of an alpha-helical coiled coil, followed by a short non-helical tailpiece at the carboxyl end. The hydrophobic amino acids show the expected seven-residue pattern a, b, c, d, e, f, g, which is modulated by a longer repeat of 28-residue zones. In addition, there are four one-residue insertions, or skip residues, at the ends of zones, at positions 351, 548, 745 and 970. Myosin is considerably less hydrophobic than tropomyosin or alpha-keratin and the outer surface of the coiled coil is covered by clusters of positive and negatively charged amino acid side-chains. Molecular models suggest that the coiled coil is continuous throughout the rod, with an approximately uniform left-handed twist, except for a few turns of helix near each skip region, where the twist flattens out to accommodate the extra residue. Fourier transforms of the amino acid profiles show strong periodicities based on repeats of seven residues (7 2 and 7 3) and 28 residues (especially 28 3 and 28 9). The positive and negative charges each have strong 28 3-residue periodicities that are out of phase with one another. The negative charges also show a 196 9-residue modulation frequency, which may reflect the presence of a 196-residue structural unit in muscle, approximately 2× 143 Å long. The distribution of charged amino acids suggests that electrostatic forces are dominant in forming the thick filament structure. Models that allow regular patterns of interacting charges are restricted and the simplest types are discussed.