Low-angle X-ray diffraction data from vertebrate muscles in the relaxed state and in the activated state at non-overlap sarcomere lengths have been used to model the relative positions of the four structural sub-domains of the actin monomer and of the tropomyosin strands and to investigate the changes that occur as a result of Ca²⁺ activation. The model is based on: (i) the published crystal structure of actin–DNase I [W. Kabsch, H. G. Mannherz, D. Suck, E. F. Pai and K. C. Holmes, Nature(London), 1990, 347, 37], (ii) the modelling of the F-actin filament [K. C. Holmes, D. Popp, W. Gebhard and W. Kabsch, Nature(London), 1990, 347, 44], (iii) published measurements of the radii of gyration of F-actin, of F-actin plus tropomyosin and of the tropomyosin strands alone, (iv) our own and published layer-line positions and intensities in the low-angle X-ray diffraction patterns from actin filaments, and (v) sensible steric constraints on actin sub-domain movements and thin filament structure. It is concluded that, even with a four sub-domain structure for actin molecules, the observed low-angle X-ray diffraction patterns cannot be explained without a substantial azimuthal swing (about 20°) of the tropomyosin strands when resting filaments are Ca²⁺-activated. The direction of this swing on Ca²⁺-activation is away from a position close to the proposed myosin binding site on actin; a result consistent with the original ‘steric blocking model’ of actin filament regulation in which tropomyosin position on actin is crucial for regulation of the myosin crossbridge cycle on actin.