The nucleotide sequence of a 7.8 kbp DNA fragment from the genome of Mycoplasma gallisepticum has been determined. The fragment contains a cluster of nine tightly linked genes coding for the subunits of the M. gallisepticum ATPase. The gene order is I (I-subunit), B (a-subunit), E (c-subunit), F (b-subunit), H (δ-subunit), A (α-subunit), C (γ-subunit), D (β-subunit) and C (ε-subunit). Two open reading frames were identified in the flanking regions; one (ORFU), preceding the I gene, encodes at least 110 amino acids and the other (ORFS), following the C gene, encodes at least 90 amino acids. The deduced amino acid sequences of the various subunits are presented and discussed with regard to the structure, function and differing sensitivity of the M. gallisepticum enzyme to dicyclohexylcarbodi-imide and aurovertin. The α- and β-subunits of the F1 portion are well conserved (51% and 65% identity with those of Escherichia coli), whereas the γ-, δ- and ε-subunits, as well as the F0-subunits, show a low percentage identity. Nonetheless, the secondary structure of the F0-subunits show a high degree of similarity to the corresponding subunits of E. coli. Two very strong potential amphipathic α-helices are predicted in the δ-subunit and the N-terminus of the b-subunit contains two hydrophobic helical stretches. The possible roles of these structural properties in the close association of the F1 and F0 multisubunit complexes among mycoplasmas are discussed.