Outer membrane phospholipase A (OMPLA) is one of the few enzymes present in the outer membrane of Escherichia coli. Two-dimensional crystals of OMPLA were grown by reconstitution of purified protein into lipid bilayers via detergent dialysis and were studied by electron crystallography. A 7.4-Angstrom projection map reveals OMPLA molecules exhibiting an oval-shaped domain of 30 x 20 Angstrom resembling the beta-barrel structure characteristic of porins, which is associated with a 25-Angstrom elongated domain of lower density.