A mutant gene for the gamma subunit of H+-translocating ATPase was cloned from Escherichia coli mutant NR70 isolated by B. P. Rosen [J. Bacteriol. 116, 1124-1129 (1973)]. Determination of its nucleotide sequence revealed a deletion of 21 base pairs between nucleotide residues 64 and 84, resulting in a deletion of seven amino acid residues (LysAlaMetGluMetValAla) from the amino-terminal portion. This deletion resulted in the loss of a hydrophobic domain of the subunit estimated by an analysis of its hydropathic character. Since F1 subunits are reported not to be assembled on the normal F0 portion of NR70, it is concluded that the hydrophobic domain deleted in the mutant subunit is important for assembly of the F1 portion. Introduction of a plasmid pNR70 carrying the mutant allele of NR70 into a wild-type strain gave no recombinants resistant to neomycin. This result suggested that the neomycin-resistant phenotype is not directly related to the defect in the gamma subunit of NR70.