Basic changes of the striated muscle fiber are described after the electron microscopy study of 200 surgical muscle biopsies from the Montevideo Neurological Institute. The inventory of these changes--true ultrastructural "signs" of muscle derangement--is presented according to the involved organelle. The value of these data is considerably enhanced by a systematic approach to the study of a muscle biopsy that should begin with the in situ examination of the muscle territory before the excision of the tissue. A throroughful histopathological study and a careful screening of semithin sections previous to the selection of areas for E.M. are also of paramount importance. The description of individual fiber lesions and that of the topography of the lesions should be accompanied by the study of vessels. nerves and muscle spindles. We stress that most of the ultrastructural findings are nonspecific. They will however acquire diagnostic value and greater interest when grouped and related to the clinical data and to the results of other diagnostic methods, besides morphology.