Lesions in striated muscle fibres from the beige mouse are described at both the light- and electronmicroscopical levels. The muscles have two types of lesions, one is well defined cores in the fibres and the other is diffusely enlarged intermyofibrillar spaces (IMS). The cores can be filled with membrane-like structures or a fluffy unstructured material. In the areas with enlarged IMS comparatively few organelles are present and the muscle fibres seem to be fragmented.