The microtubule-associated protein tau is a major cytoskeletal protein involved in the neurofibrillary tangles of Alzheimer's disease. Although tau is predominantly a neuronal protein, it has been demonstrated in glia and other nonneuronal cells. We describe the presence of microtubule-associated protein tau epitopes in various muscle fiber lesions in oculopharyngeal and Becker muscular dystrophy, dermatomyositis, central core disease, neurogenic atrophy, and in the recovery phase of an attack of malignant hyperthermia. Western blot demonstrated a 100- to 110-kd tau-immunoreactive protein probably corresponding to 'big tau' as described in peripheral nerves. Tau immunoreactivity in muscle fiber lesions usually co-localized with tubulin, although electron microscopy failed to show an increase in microtubules. Tau and tubulin reactivity also correlated with the presence of desmin and vimentin epitopes. Possible explanations for the presence of tau are briefly discussed.