A Fuji imaging plate and synchrotron radiation are the most distinct innovations of the last twenty years in the X-ray diffraction experiments on biological materials. Here we present results of recent experiments on skeletal muscles made at Photon Factory, Tsukuba. It is now possible to record a two-dimensional X-ray diffraction pattern from a rabbit or frog single skinned fiber with a 30-sec exposure. Although weaker compared with those from whole muscles, it shows layer-lines up to 5.1 nm. When the fiber is activated by Ca2+, the pattern changes in a way similar to that observed when a live muscle is electrically stimulated. Use of single fibers makes various types of structural experiments much easier than using whole muscles or fiber bundles. Not only suitable for physiological experiments, better diffusion makes it also suitable for biochemical experiments using various kinds of labels. Time-resolved experiments with imaging plates are possible by using an imaging-plate exchanger devised by Dr. Y. Amemiya. By combining this and a fast-acting mechanical shutter, it is possible to record a two-dimensional diffraction pattern from a frog whole muscle shortening at the maximum speed. The pattern thus obtained shows weakening of the 5.1 and 5.9-nm actin layer-lines and the third (14.3 nm) and the sixth (7.2 nm) myosin meridional reflections, compared with the pattern from isometrically contracting muscles. On the other hand, the second meridional reflection from the thick filament is intensified. These results suggest very different arrangement of myosin heads during active shortening from that during isometric contraction.