Giant viruses are a highly diverse group of double-stranded DNA viruses that infect unicellular eukaryotes. Mimiviruses contain a 1.2 Mb genome, and the virions consist of icosahedral capsids that are surrounded by a dense layer of radially arranged fibrils. Although structural analyses have provided insights into the capsid structure, the internal organization of the virion core is elusive. Villalta et al. used cryo-electron microscopy, cryo-electron tomography and proteomics to determine how the mimivirus packages and protects its genome. The genome is encased into a ~30-nm diameter helical protein shell that is composed of two oxidoreductases, which also comprise the fibrils that decorate the mimivirus capsid. The genome is arranged into 5- or 6-start left-handed super-helices. In addition, RNA polymerase subunits are enriched in the nucleoprotein fibre, possibly providing a strategy for packaging and protecting the viral genome in a state ready for immediate transcription upon unwinding in the host cytoplasm.