Crystalline sheets of the 50S ribosomal subunits of Escherichia coli have been formed in vitro. Electron micrographs of these arrays diffract to 35-angstrom resolution. The lattice parameters of the crystals are a = 330 +/- 20 angstroms, b = 330 +/- 30 angstroms, and alpha = 123 degrees +/- 5 degrees, and the space group is most likely p21. These arrays of ribosomal subunits are sufficiently ordered to resolve such known features of the large ribosomal subunit as the L7/L12 stalk and the central protuberance.