AbstractMitochondrial crystalline inclusions, frequently found in mitochondrial myopathies, were analyzed by crystallographic techniques and computer‐aided image processing. It could be shown that these structures were real crystals. There are two distinct types of crystal, which can be distinguished by shape, size, and pattern. So‐called type I crystals are usually present in the intracristal space, whereas the type II crystals are preferentially located in the intermembrane space between outer and inner mitochondrial membranes. The unit cell dimensions were found to be 38 × 34 × 8 nm for the type I crystals and 20 × 17 × 8 nm for the type II crystals. These results strongly suggest that the crystals are composed of macromolecules, presumably proteins. Arguments are presented that indicate that type I crystals occur only in type 1 muscle fibers and type II crystals in type 2 muscle