In Escherichia coli, the F1 sector of the F1F0-ATP synthase is connected to the membrane-embedded F0 sector by a narrow stalk, thought to be formed by subunits delta and b. Mutagenic analysis was used here to study the structure and function of subunit delta. First, random mutations in the protein were generated by bisulfite mutagenesis. Two single missense mutations causing impaired growth by oxidative phosphorylation were found, namely delta A149T and delta G150D. Both occur at the conserved C-terminal region, which has been suggested previously to be functionally important. Two techniques were applied to study the C-terminal region in greater detail. Cassette mutagenesis was used to randomly mutate the sequence from delta 145 to delta 167, and residues delta A149 and delta G150 were specifically mutated by site-directed mutagenesis to obtain multiple substitutions at each position. Fifteen of the residues between delta 145 and delta 167 were mutated. None was found to be absolutely essential for function. However, the properties of the mutants obtained, which included partial impairment of growth by oxidative phosphorylation, temperature sensitivity, and specific structural requirements at residues delta A149 and delta G150, confirmed that this region is important for enzyme function. Based on these studies, and on secondary and tertiary structure predictions, a model for subunit delta and its orientation in F1F0-ATP synthase is proposed.