Rhodobacter sphaeroides swims by unidirectional rotation of a single medial flagellum, re-orienting randomly by Brownian motion when flagellar rotation stops and restarts. Previously we identified a mutant with a paralysed flagellum, which was complemented by a Rhodobacter gene that had homology to motB of Escherichia coli, a bacterium with bidirectional flagella. In the current work, interposon mutagenesis upstream of the Rhodobacter motB gene gave rise to another paralysed mutant, RED5. DNA sequence analysis of this upstream region showed one open reading frame, the predicted polypeptide sequence of which shows homology to the MotA protein of E. coli. MotA is thought to be a proton 'pore' involved in converting proton-motive force into flagellar rotation. Several potential proton-binding amino acids were conserved between putative membrane-spanning regions of R. sphaeroides and E. coli MotA sequences, along with a highly charged cytoplasmic linker region. Complementation studies with mutant RED5 showed the presence of an active promoter upstream from motA which was found to be necessary for expression of both motA and motB. Examination of the upstream DNA sequence showed only one putative promoter-like sequence which resembled a sigma 54-type promoter, including a potential enhancer binding site. The overall similarities between the R. sphaeroides MotA protein and those from other bacteria suggest that, despite the novel unidirectional rotation of the R. sphaeroides flagellum, the function of the MotA protein is similar to that in bacteria with bidirectional flagella.