Chlamydiae contain two porins, MOMP and PorB, that facilitate diffusion of solutes through the outer membrane. MOMP is a general porin that permits the diffusion of a wide variety of compounds including carbohydrates and amino acids. The relative inefficiency of PorB as a general porin and its low abundance in the outer membrane suggest that it may function as a substrate-specific porin. The tricarboxylic acid (TCA) cycle of chlamydiae is incomplete and to function would require the exogenous acquisition of 2-oxoglutarate or glutamate. A liposome-swelling assay for anions as well as an enzyme-linked liposome assay were used to demonstrate the efficient diffusion of dicarboxylates such as 2-oxoglutarate through PorB. These data demonstrate that PorB is a dicarboxylate-specific porin that may feed the chlamydial TCA cycle and provide chlamydiae with carbon and energy production intermediates.