Gram-negative bacteria have evolved transport complexes that export macromolecules and toxic substances across the two membranes of the cell envelope in a single energy coupled step. The process requires (1) a cytoplasmic membrane export system, (2) a membrane fusion protein (MFP), and (3) an outer membrane factor (OMF). Families comprising the former two constituents have been described previously. We here present an analysis of the phylogenetic and structural characteristics of the OMF family. Twenty-one members of this family have been identified, and based on available evidence, they function in conjunction with ABC, RND, MFS and/or other types of cytoplasmic transport systems. OMFs exhibit fairly uniform sizes (398-495 residues with two exceptions), and based on computational analyses, they may form beta-barrel structures consisting of up to 16 beta-strands. Phylogenetic analyses reveal that while the MFPs cluster in accordance with the type of cytoplasmic membrane transport systems with which they function, OMFs do not. We conclude that OMFs probably comprise a family of outer membrane porin-type proteins of uniform structure which did not coevolve with their cognate cytoplasmic membrane transport systems.