Buchnera aphidicola is an intracellular, non-cultivable prokaryotic symbiont of the aphid Schizaphis graminum. A 6.8-kilobase fragment from B. aphidicola was cloned and sequenced and was found to contain genes encoding for proteins of the ATP synthase. The order of the genes, atpBEFHAGDC, is identical to that found in Escherichia coli and many other prokaryotes. This genetic organization is different from that observed in organelles such as mitochondria and chloroplasts, in which the genes are partitioned between the organellar and nuclear genomes, One difference between B., aphidicola and E. toil was the absence of aryl, a gene of unknown function, which in E. coli precedes atpB. As is the case of many other prokaryotes, atpBEFHAGDC appears to constitute a single transcription unit. The detection in B. aphidicola of the genes encoding the ATP synthase as well as past observations indicating that this organism is capable of respiration are consistent with the utilization by B. aphidicola of a proton gradient for the generation of ATP.