Scanning force microscope images of membrane-bound Escherichia coli ATP synthase F0 complexes have been obtained in aqueous solution. The images show a consistent set of internal features: a ring structure which surrounds a central dimple and contains an asymmetric lateral mass. Images of trypsin-treated F0 complexes, which have lost part of their b subunits, show a reduced asymmetric mass, while images of c-subunit oligomers, which lack both the a and b subunits, show a ring and dimple but do not have an asymmetric mass. These results support models in which the F0 complex contains a ring of 9-12 c subunits with the b subunits located outside this ring, and show that scanning force microscopy is able to provide structural information on membrane proteins of molecular mass less than 200,000 Da.