X-ray diffraction studies of actin-containing thin filaments from frog skeletal muscles were done using a new type of integrating X-ray area detector (an imaging plate) and synchrotron radiation. The high sensitivity and wide dynamic range of the imaging plate made it possible to clearly record the weak thin filament pattern from a muscle during isometric contraction in less than 20 sec exposure time. The intensity distributions of the actin-based layer lines were measured. During contraction, most of the actin-based layer lines increased in intensity without noticeable changes of their layer-line profile and axial spacing in the resting state. The difference cylindrically symmetrical Patterson function was calculated using the intensity data of 15 actin-based layer lines. During contraction, the Patterson map was quite different from the rigor map and no significant indication of binding of the myosin heads was detected in the contracting map. This revealed that the labeling of myosin heads was not in accordance with the actin symmetry. Assuming that the intensity change during contraction is due to the structural change occurring within the thin filaments by the interaction with the myosin heads, model-calculating studies were done to interpret the intensity change. The observed intensity increase could be explained by changing the structure of the actin subunit and the position of tropomyosin molecules in the thin filament. With reference to the three-dimensional reconstructions of the thin filament electron micrographs, some models of the thin filament in the resting and contracting states were presented.