The molecular shape of dystrophin has been reported to be a 175 nm flexible rod [Pons, F. et al. (1990) Proc. Natl. Acad. Sci. USA 87, 7851-7855] or a 120 nm dumbbell [Murayama, T. et al. (1990) Proc. Jpn. Acad. 66B, 96-99]. The present work revealed that 100 nm flexible rods with or without spheres were predominant in highly purified dystrophin preparations. When the sample was subjected to gel filtration, dystrophin oligomers were isolated just after the void volume and the fraction largely consisted of dumbbell-shaped molecules. From various rotary-shadowed images, it was suggested that dystrophin is a rod with spheres at both ends, approximately 110 nm long and 2 nm wide. It appeared that this monomer binds to another monomer in a staggered way, forming a dimer, and the dimers associate with each other side-by-side, forming a dumbbell-shaped tetramer, 130 nm long and 5 nm wide. The tetramers form an end-to-end aggregate. It seemed that the dumbbell structure was not affected by alkaline (pH 11) treatment to dissociate dystrophin associated glycoproteins, but was deteriorated by detergent, NP-40, Triton X-100, or CHAPS, used for solubilization of membrane-bound dystrophin.