How the `energy currency' of the cell, adenosine triphosphate (ATP), is produced consequent upon the oxidation of foodstuffs (oxidative phosphorylation) is, despite prolonged research, still a matter of debate and the molecular mechanism of the process is unknown. It appears that the problem of oxidative phosphorylation can be approached with the aid of the biochemical genetics of the bacterium Escherichia coli. The ease of manipulation of bacteria and definitive results obtained by this approach have been invaluable in solving other major biochemical problems. Mutants affected in oxidative phosphorylation have been isolated and characterized by genetic and biochemical techniques. These `unc' mutants are affected in the adenosine triphosphatase (ATPase) multiprotein complex which is part of the cell membrane and responsible for the terminal stages of ATP synthesis. Seven distinct genes concerned with oxidative phosphorylation have been characterized in E. coli and shown to be part of an operon. The relationships between the different classes of unc genes and the various components of the ATPase have been established. Information about the assembly of the ATP synthesizing complex in the cell membrane has also been obtained and the stage set for further studies on the assembly, control and function of the ATP synthesizing system.