Respiration in all cells depends upon synthesis of ATP by the ATP synthase complex, a rotary motor enzyme. The structure of the catalytic moiety of ATP synthase, the so-called F(1) headpiece, is well established. F(1) is connected to the membrane-bound and ion translocating F(0) subcomplex by a central stalk. A peripheral stalk, or stator, prevents futile rotation of the headpiece during catalysis. Although the enzyme functions as a monomer, several lines of evidence have recently suggested that monomeric ATP synthase complexes might interact to form a dimeric supercomplex in mitochondria. However, due to its fragility, the structure of ATP synthase dimers has so far not been precisely defined for any organism. Here we report the purification of a stable dimeric ATP synthase supercomplex, using mitochondria of the alga Polytomella. Structural analysis by electron microscopy and single particle analysis revealed that dimer formation is based on specific interaction of the F(0) parts, not the F(1) headpieces which are not at all in close proximity. Remarkably, the angle between the two F(0) part is about 70 degrees, which induces a strong local bending of the membrane. Hence, the function of ATP synthase dimerisation is to control the unique architecture of the mitochondrial inner membrane.