Ultrastructural study of muscles taken from a mutant (LWC) strain of Japanese quail with myotonia showed type 2 fibre atrophy, ring fibre formation, sarcoplasmic masses, and "moth-eaten" fibres. In these abnormal fibres, the most characteristic feature was the loss of interconnection among the myofibrils, mitochondria, and T tubules. Apparently normal muscle fibres often showed mild changes, such as proliferation of T tubules and enlarged sarcoplasmic areas with increased glycogen granules and ribosomes at the periphery of the fibres. The study suggested that one possible cause of these ultrastructural changes was a defect in cytoskeleton of muscle cells, especially in intermediate filaments.