Cryo-electron microscopy (cryo-EM) is rapidly emerging as a powerful tool for protein structure determination at high resolution. Here we report the structure of a complex between Escherichia coli beta-galactosidase and the cell-permeant inhibitor phenylethyl beta-D-thiogalactopyranoside (PETG), determined by cryo-EM at an average resolution of similar to 2.2 angstroms (angstrom). Besides the PETG ligand, we identified densities in the map for similar to 800 water molecules and for magnesium and sodium ions. Although it is likely that continued advances in detector technology may further enhance resolution, our findings demonstrate that preparation of specimens of adequate quality and intrinsic protein flexibility, rather than imaging or image-processing technologies, now represent the major bottlenecks to routinely achieving resolutions close to 2 angstrom using single-particle cryo-EM.