The Na⁺ F₁F₀ ATP synthase operon of the anaerobic, acetogenic bacterium Acetobacterium woodii is unique because it encodes two types of c subunits, two identical 8 kDa bacterial F ₀-like c subunits (c₂ and c₃), with two transmembrane helices, and a 18 kDa eukaryal V₀-like (c₁) c subunit, with four transmembrane helices but only one binding site. To determine whether both types of rotor subunits are present in the same c ring, we have isolated and studied the composition of the c ring. High-resolution atomic force microscopy of 2D crystals revealed 11 domains, each corresponding to two transmembrane helices. A projection map derived from electron micrographs, calculated to 5 Å resolution, revealed that each c ring contains two concentric, slightly staggered, packed rings, each composed of 11 densities, representing 22 transmembrane helices. The inner and outer diameters of the rings, measured at the density borders, are approximately 17 and 50 Å. Mass determination by laser-induced liquid beam ion desorption provided evidence that the c rings contain both types of c subunits. The stoichiometry for c₂/c₃ : c₁ was 9 : 1. Furthermore, this stoichiometry was independent of the carbon source of the growth medium. These analyses clearly demonstrate, for the first time, an F₀-V₀ hybrid motor in an ATP synthase.