The structure of the non-coding region of Xenopus laevis mitochondrial DNA has been studied by electron microscopy analysis of DNA molecules end-labelled with streptavidin-ferritin. We have shown that the effect of a protein modifying the shape of the DNA double-helix can be studied and precisely located by this method. It was found that the non-coding region contains curved segments and that the mitochondrial protein mtDBP-C preferentially enhances the curvature of the promoters-replication origin region.