Electron microscopic study of a rhabdomyoma of the soft palate of a 50-year-old woman revealed linear structures within the intracristal spaces of mitochondria. In the center of most cristae, an electron-dense line was found located in parallel arrangement to the limiting cristal membrane. The linear inclusion was separated from the cristal membrane by a narrow space of lower electron-density. Thus, when sectioned longitudinally, the abnormal cristae exhibited a pentalaminar appearance. Additionally, they were characterized by a greater diameter than that of normal cristae and a "rigid" configuration. Higher magnification showed that the intracristal line consisted of a row of periodically arranged, electron-dense dots. Sometimes, these seemed to be connected with the wall of the intracristal space by fine, electron-dense cross-striations. In some mitochondria of rhabdomyoma cells, crystals with a linear substructure were observed in both intra- and extracristal spaces. The crystalline bodies located within cristae had probably been formed by accumulation of linear inclusions. The extracristal bodies seemed to have arisen from fusion of pentalaminar cristae. It is supposed that the intracristal linear inclusions in mitochondria of rhabdomyoma cells represent crystallized proteins of mitochondrial enzymes.