Although mitochondria derive from alpha-proteobacteria, many proteins acting in this organelle did not originate from bacteria. In particular, phylogenetic evidence indicates that RNA polymerase, DNA polymerase and DNA primase--with homologues encoded by T3/T7-like bacteriophages--have replaced the ancestral proteins of bacterial origin. To date, there was no clear explanation for this puzzling observation. Bacterial genomics has now revealed the presence of cryptic prophages that are related to T3/T7 in several genomes of proteobacteria. We propose that such a prophage was present in the ancestral alpha-proteobacterium at the origin of mitochondria and that RNA polymerase, DNA polymerase and DNA primase encoded by this prophage replaced the original bacterial enzymes to function in mitochondria. Another T3/T7 viral-like RNA polymerase is functional in the chloroplast, indicating that a strong selection pressure has favored replacement of some cellular proteins by viral proteins in organelle evolution.