We have sequenced the entire mitochondrial DNA (mtDNA) from a 54-year-old man with chronic progressive external ophthalmoplegia (PEO) and hyperCKemia. Muscle biopsy showed ragged red and SDH positive/COX negative fibres, and the biochemistry was suggestive mitochondrial respiratory chain dysfunction. Analysis of mtDNA revealed a heteroplasmic m. 4308G>A mutation in the transfer RNA isoleucine gene (MT-TI gene). Our report expands the genetic heterogeneity of PEO.