The skeletal muscle of Japanese quails with acid maltase deficiency (AMD) was studied morphologically at various developmental stages, from the 16th embryonal day up to 3 months after hatching. Membrane-bound glycogen particles began to appear in the affected skeletal muscle at the 16th embryonal day. In normal embryonic muscles, a certain amount of free glycogen particles was observed but they were not membrane-bound. Therefore, this is the earliest morphological event in the muscle of Japanese quails with AMD. In muscle at 3 weeks after hatching, the initial focal degeneration of myofibrils was recognizable but it was not associated with autophagic vacuoles. Quails with AMD developed muscle weakness and difficulty in lifting their wings at about 3 months after hatching: then numerous autophagic vacuoles were present. The formation of large autophagic vacuoles followed by fiber loss and fatty replacement seemed to contribute to the progressive muscle weakness. The study of Japanese quail with AMD will greatly facilitate the elucidation of the pathogenetic mechanism and is also a useful model for therapeutic trials in human AMD.