Native and recombinant FomA proteins were extracted by detergent from the cell envelopes of Fusobacterium nucleatum and Escherichia coli, and purified to near homogeneity by chromatography. Circular dichroism analysis revealed that the FomA protein consists predominantly of beta-sheets, in line with the previously proposed 16-stranded beta-barrel topology model. Results obtained by trypsin treatment of intact cells and cell envelopes of F. nucleatum, and from limited proteolysis of purified FomA protein, indicated that the N-terminal part of the FomA protein is not an integral part of the beta-barrel, but forms a periplasmic domain. Based on these results a new topology model is proposed for the FomA protein, where the C-terminal part forms a 14-stranded beta-barrel separate from the periplasmic N-terminal domain.