The membrane topology of a resistance-nodulationdivision (RND) family transporter, MexD of Pseudomonas aeruginosa, was determined. Although it had been predicted previously that most RND proteins contain 12 transmembrane helices, three independent computer programs used in the present study predicted that MexD possessed 11, 14 or 17 transmembrane segments. To investigate the topology of MexD more thoroughly, 25 MexD-PhoA (alkaline phosphatase) and 18 MexD-Bla (L L-lactamase) fusion plasmids were constructed and analyzed. The resulting topological model had just 12 transmembrane helices and two periplasmic loops of about 300 residues between helices 1 and 2 and helices 7 and 8. It is therefore proposed that the N-and C-termini are located in the cytoplasm and the predicted orientation is consistent with the`positiveinside rule'. This topological model can be applied to other RND proteins.z 1999 Federation of European Biochemical Societies.