A non-familiar case of Kiloh-Nevin ocular myopathy with important histochemical and ultrastructural abnormalities is reported. The patient, a 43 year-old male presented with 10 year long, progressive ocular ophtalmoplegia, myasthenic symptoms and severe muscular pain, an uncommon finding in this type of myopathy. The histochemical study showed muscular atrophy, mainly in type I fibres and a great amount of ragged-red fibres. Of particular interest, in this case, was the ultrastructural finding of severe mitochondrial abnormalities with a lot of inclussions in the cristae matrix; nevertheless these changes are not specific. The authors suggest a possible abnormality in the neuro-muscular transmission system and a genetically controlled enzimatic factor as responsible for the aethiology and pathogenesis of the Kiloh-Nevin ocular myopathy.