Solemya reidi, a gutless clam found in sulfide-rich habitats, contains within its gills bacterial symbionts thought to oxidize sulfur compounds and provide a reduced carbon food source to the clam. However, the initial step or steps in sulfide oxidation occur in the animal tissue, and mitochondria isolated from both gill and symbiont-free foot tissue of the clam coupled the oxidation of sulfide to oxidative phosphorylation [adenosine triphosphate (ATP) synthesis]. The ability of Solmya reidi to exploit directly the energy in sulfide for ATP synthesis is unprecedented, and suggests that sulfide-habitat animals that lack bacterial symbionts may also use sulfide as an inorganic energy source.