A bit of protoplasm represented by a single living individual of any type of protozoa has been continuously living since the beginnings of life and has the possibility of an indefinitely continued existence in the future. The factors controlling this longevity are: (1) Suitable environment and food. (2) Metabolic activities of the individual. (3) Catalytic processes of cell division whereby a changed organization due to metabolism is restored, in part, to the fundamental organization of the species from which new derived structures are developed. (4) Catalytic processes of endomixis whereby a changed organization due to continued metabolism is more completely restored to the fundamental organization from which new derived structures, including the macronucleus, are formed. (5) Catalytic processes of conjugation affecting the entire organization beginning with the cortex, whereby an organization changed by metabolism is fully restored to the fundamental organization from which a complete new set of derived structures including the macronucleus is formed. Conjugation merotomy shows that amphimixis is not necessary for continued vitality. (6) Encystment, associated in most cases with endomixis, whereby the fundamental organization in a dried state may withstand unfavorable environmental conditions for long periods. Animal flagellates retain protoplasmic vitality indefinitely with only the first 3 of the above factors.