The two-empire system is the top-level biologicial classification system in general use before the establishment of the three-domain system. Some biologists prefer the two-empire system over the three-domain system, in that the three-domain system overemphasizes the division between Archaea and Bacteria. Detractors of the two-domain system point to evidence that suggest that the Archaea are more closely related to the Eukarya than the Bacteria.
|Two empires||Three domains||Six kingdoms|
| Biota / Vitae |
|Acytota / Aphanobionta (Viruses, Viroids, Prions?, ...) non-cellular life|
| Cytota |
| Prokaryota / Procarya |
|Eukaryota / Eucarya||Protista|