The main ultrastructural findings in a muscle biopsy from a child aged 11 months with a GM1 gangliosidosis were cytoplasmic inclusions of two different types: (1) inclusions filled with a moderate electron dense and polymorphous material thought to correspond to ganglioside accumulation and lying only in the Schwann cells of intramuscular nerves. (2) Vacuolar inclusions regarded as containing polysaccharides and observed in perineurial cells, endothelium and pericytes of blood vessels, and also in muscle satellite cells. The muscle fibres only exhibited moderate and non-specific changes. The study shows that in a muscle biopsy of GM1 gangliosidosis the two characteristic types of storage deposits and their preferential localization in different cells may be demonstrated, providing that the intramuscular nerves and motor end plates are examined.