The experiment was designed to investigate type-selectivity of the involved muscle fibers and changes in the component ratios of the fiber types following the progression of disuse atrophy in the skeletal muscle. After inducing disuse atrophy of the skeletal muscle by applying cast on the hind limb, we used histological and histochemical stains in the sections of the rat Tibialia anterior muscles. The results were as follows. 1. Even 8 weeks after immobilization of the hind limb, differentiation of muscle fiber types by histolo gical and histochemical staining methods in the Tibialis anterior muscle could be possible. 2. Atrophy of muscle fibers was more pronounced in type IIB and type I fibers than in type IIA fibers. 3. Central migration of sarcolemmal nuclei and ring fiber appeared after 6th and 8th weeks of immobilization respectively, in the H&E and trichrome preparations. Ac-pase or Alk-pase positive fibem were not noted throughout the experimental periods.