We have analyzed the known three-dimensional structures of trimeric porins from bacterial outer membranes. The distribution of surface-exposed residues in a direction perpendicular to the membrane is similar to that in helical membrane proteins, with aliphatic residues concentrated in the central 20 A of the bilayer. Outside these residues is a layer of aromatic residues, followed by polar and charged residues. Residues in the trimer interface are more conserved than residues not in the interface. By comparing the interface and noninterface residues, an interface preference scale has been derived that may be used as a basis for predicting interface surfaces in monomer models.