Examination by light and electron microscopy of more than 100 muscle biopsies revealed one very unusual case. A 4-year-old boy with non-progressive muscle weakness and hypotonia was found to have small particles, termed "myogranules", in many muscle fibres from two gastrocnemius biopsies. Paraffin sections and thin sections of plastic-embedded muscle showed that the rod-shaped myogranules measured between 0.1 and 5 microns in length, and were usually orientated in the long axis of the fibre. Normal cross-striations could not be seen in areas occupied by myogranules, although adjacent parts of the same fibre were normal. Electron micrographs showed myofilaments running through the myogranules and a periodicity similar to sections of recrystallized muscle protein paramyosin. It is possible that this child has a disturbance of muscle proteins.