A Fourier analysis of the distributions of different types of amino acids in the sequence of tropomyosin shows strong 14th-order peaks in the profiles of both negatively charged and non-polar amino acids, with a period of 1923 residues and an overall repeat length of 275 ± 2 amino acids, which is shorter than the sequence length of 284 amino acids. Both peaks are statistically significant and confirm Parry's work (1974, 1975b). The regularities are analysed in terms of an assumed supercoil structure in which two α-helices lie parallel and in register to form a supercoil with a pitch of 137 A. These molecules are then assumed to overlap end-to-end by eight to nine amino acids so that the periodicity is continuous along an extended filament of linked tropomyosin molecules. The periodic features are stronger in the outer surface of the molecule away from the core of the supercoil. The sequence divides into 14 bands which each have a narrow zone of net positive charge and a broader negatively charged zone. Overlapping every positive zone is a hydrophobic zone which always has at least one non-polar group on the outer surface. Anomalies in the charge distribution are found near the molecular ends and close to Cys190. These are attributed to the end-to-end overlap site and the troponin binding site. In the thin filament the 137 A pitch supercoil would make seven half-twists relative to the twisted actin helix along a 385 A length, so that a pair of adjacent bands would be oriented equivalently with respect to a pair of actins 28 A apart. We therefore suggest that the bands (each containing one zone of each type) should be divided alternately into two series, α and β. Every pair of bands is 3913 residues long and each of the seven pairs corresponds with one segment of the 42-residue gene duplication repeat observed previously in the sequence. The disparity between the periods of 42 and 3913 is overcome by deletions and insertions. The 3913-residue periodicity is not simply a consequence of the supercoil structure or gene duplication but is probably a result of adaptation to the spatial periodicity of the actin helix in muscle. Although the α and β bands are alike in general, they differ systematically in detail and the α bands are more regular than the β. We propose that the seven α and seven β bands are alternative sets of sites which bind equivalently to complementary groups of sites on seven actins in the “relaxed” and “active” states of muscle, respectively. In each band the negative zone probably attaches to actin by magnesium bridges and the hydrophobic zone by direct contacts with the narrow outer edge of the supercoil. Since the supercoil twists 90 ° relative to actin on passing between adjacent α and β bands, a quarter rotation of the whole tropomyosin molecule would detach one set of seven sites and attach the other, allowing a highly co-operative switch mechanism.