Many bacteria have primitive proteinaceous organelles that are known as bacterial microcompartments (BMCs). All BMCs share a common architecture, with a selectively permeable protein shell that encapsulates different metabolic enzymes. Although detailed structural information is available for the single components of the shell, how the shell assembles was unknown. In a new study, Sutter et al. determined the crystal structure of an intact shell from Haliangium ochraceum, thereby revealing the basic principles of its assembly. In particular, by using a recombinant system that contained selected components of the shell, the authors generated a complete 40 nm shell with a molecular mass of 6.5 MDa and determined its structure to a resolution of 3.5 Å. Considering that these structures are widely conserved in bacteria, these findings probably apply to various functionally distinct bacterial organelles.