Five proteins (MotA, MotB, FliG, FliM, and FliN) have been implicated in energizing flagellar rotation in Escherichia coli and Salmonella typhimurium. One model for flagellar function envisions that MotA and MotB comprise the stator of a rotary motor and that FliG, FliM, and FliN are part of the rotor. MotA probably functions as a transmembrane proton channel, and MotB has been proposed to anchor MotA to the peptidoglycan of the cell wall. To study interactions between the Mot proteins themselves and between them and other components of the flagellar motor, we attempted to isolate extragenic suppressors of 13 dominant or partially dominant motB missense mutations. Four of these yielded suppressors, which exhibited widely varying efficiencies of suppression. The pattern of suppression was partially alleles-specific, but no suppressor seriously impaired motility in a motB+ strain. Of 20 suppressors from the original selection, 15 were characterized by DNA sequencing. Fourteen of these cause single amino acid changes in MotA. Thirteen alter residues in, or directly adjacent to, the putative periplasmic loops of MotA, and the remaining one alters a residue in the middle of the fourth predicted transmembrane helix of MotA. We conclude that the MotA and MotB proteins form a complex and that their interaction directly involves or is strongly influenced by the periplasmic loops of MotA. The 15th suppressor from the original selection and 2 motB suppressors identified during a subsequent search cause single amino acid substitutions in FliG. This finding suggests that the postulated Mot-protein complex may be in close proximity to FliG at the stator-rotor interface of the flagellar motor.