Low-angle X-ray diffraction diagrams have been recorded from frog sartorius muscles with a time resolution of a few milliseconds, using synchrotron radiation. The layer-line reflections, believed to arise from the myosin cross-bridges, with an approximately helical repeat of 429 Å and a subunit repeat of 143 Å, change in characteristic ways during the contractile cycle. The off-meridional parts of the pattern show a large decrease in intensity which, to a first approximation, follows the same time-course as the onset and decay of tension when unfatigued muscles are used. More precisely, the initial decrease in intensity of the first layer-line reflection (429 Å) runs about five to ten milliseconds ahead of tension development (at 10 °C) and the recovery of intensity either coincides with the tension decay or sometimes is delayed 10 to 20 milliseconds behind it. A similar synchrony between tension and intensity change is seen at 5 °C and 2 °C, even though the absolute time-scales involved may be several times longer. The intensity of the first layer-line reflection falls during contraction to about 20% of the value in the resting muscle. The time-course of the changes seen on the off-meridional parts of the other layer-lines was very similar; on the third layer-line, for example, the return of intensity during relaxation was just as fast as on the first layer-line. However, the drop of intensity on the third layer-line was only to about 30 to 40% of the value in the resting muscle. No sign of any additional layer line reflections, for example those characteristic of rigor muscles, was detected during contractions. The intensities of the meridional reflections exhibited somewhat different behaviour. The intensity of the strong 143 Å meridional reflection fell during the onset of contraction, then recovered towards or even beyond its resting value during the time that muscle tension was high, and then fell substantially again during relaxation, returning to its resting value with a delay of several hundred milliseconds behind tension decay. The 215 Å (“forbidden”) meridional reflection became very weak during the onset of tension, but remained at this level during the whole period of contraction, and then recovered again with a substantial delay behind tension decay. The width of the 143 Å reflection across the meridian increased substantially during contraction, indicating (since it is strongly sampled) a decrease in the extent of filament alignment. The width of the reflection returned to its resting value with a delay behind tension, and this accounts in large part for the low intensity of the meridional reflections during relaxation, when the offmeridional reflections have already recovered most of their resting intensity. When these changes in width are taken into account, it is found that, with rare exceptions, freshly dissected muscles always show a substantial increase in the integrated intensity of the 143 Å meridional reflection during the peak of both tetani and twitches. The results indicate a close correlation between the helical disordering of crossbridges and tension development. The behaviour of the meridional reflections suggests that the disorder has a larger effect on the circumferential positioning of the cross-bridges than on the regularity of their axial repeat.