Two Japanese Black bull calves from a dam showed muscular weakness and became recumbent after birth. At necropsy, skeletal muscles, including face, neck, body and proximal and distal forelimb and hindlimb were extremely pale in color and edematous. Histopathological examination of skeletal muscles revealed degenerative changes as follows: replacement of muscle with fat, variation in muscle fiber diameter, internal nuclei, central core-like structures, and vacuolar and hyaline degeneration of muscle fibers. Ultrastructurally, the lesions were characterized by focal myofibrillar disorganization with streaming or irregularity of Z bands. The present cases seem to be classified as congenital myopathy based on pathological alterations and age of onset.