The DNA encoding the subunits of the ATP synthase (F 1 F o ) of Streptomyces lividans 66 strain 1326 was identified using oligodeoxyribonucleotide probes derived from the N-terminal sequence of subunit γ of the F 1 complex. The complete nucleotide sequence of the operon was determined. The atp operon contains nine genes, atpIBEFHAGDC, encoding the eight structural components of the ATP synthase complex and the i protein, a polypeptide of unknown function. The gene order found is identical to that in other non-photosynthetic eubacteria. The determination of the N-terminal amino acid (aa) sequences of the F 1 subunits α, β, γ, δ and allowed us to identify the translational start points and to define the primary structures of the proteins. The aa sequence deduced for subunit δ revealed an N-terminal extension of about 90 aa, which is not present in any δ subunit or OSCP (oligomycin sensitivity-conferral protein) of other species studied so far. The phylogenetic relationship of eu- and archaebacteria was investigated using sequencing data of the highly conserved β subunit of different ATP synthases including that of S. lividans. The calculations revealed that S. lividans β does not form a phylogenetic group together with the Gram + taxa of low G + C contents, but is more closely related to the β subunit of Rhodobacteria.