Archezoa was a kingdom proposed by Thomas Cavalier-Smith for eukaryotes that diverged before the origin of mitochondria. At various times, the pelobionts and entamoebids (now Archamoebae), the metamonads, and the Microsporidia were included here. These groups appear near the base of eukaryotic evolution on rRNA trees. However, all these groups are now known to have developed from mitochondriate ancestors, and trees based on other genes do not support their basal placement. The kingdom Archezoa has therefore been abandoned.
Archaezoa is composed of two kingdoms of protists, Kingdom Diplomadida and Kingdom Parabasala. These two kingdoms are grouped together because they lack mitochondria. The Archaezoa hypothesis suggests that these two kingdoms originally had mitochondria, but lost them before mitochondria became symbionts of protists. This lineage is believed to be the proof of Eukaryotic Endosymbiosis. Molecular evidence indicates that Archaezoa have the genetic marker of mitochondria in their nucleus that suggests they had and then lost mitochondria. Both of these kingdoms are parasites, as they have to acquire ATP from some source. An example of these Archaezoans is Trichomonas vaginalis, a common urinary infection that is transmitted through sexual contact.