1. Prorodon griseus is a Holotrichous ciliate. It is recognized by its sub-terminal oral aperature, its rod like structure enclosing the pharynx, and its terminal contractile vacuole. 2. P. griseus has a single micronucleus and a single macronucleus. 3. Binary fission and conjugation occur either while encysted or in the free forms. 4. Encystment serves for protection, reproduction and reorganization. 5. Cysts are of two kinds: (a) temporary for protection during short intervals. (b permanent for reorganization of cell and protection for long intervals, from one cycle to the next. 6. Conjugation is terminal, fusion occurring at their anterior ends. Conjugation occurs immediately after the animals escape from the cysts. 7. The animals are found encysted during the greater part of their life history. The greater number of free forms are found immediately after conjugation, during the active feeding period. 8. Encystment and conjugation in P. griseus have little or no effect in the production of new vigor and in the change of fission rate. 9. The old macronucleus persists during early conjugation, then disintegrates and disappears by absorption within the cytoplasm. 10. The new micronucleus and macronucleus are formed from the daughter nuclei, produced by a single division of the conjugation nucleus. 11. The reduction of chromosomes occurs in the second maturation division by a pairing of the eight chromosome in the formation of the four bivalent structures. 12. In the third division the pronuclei at first are connected by a drawn out fiber. 13. The pronuclei elongate slightly during their process of fusion. 14. In cellular reorganization within the permanent cyst, the cilia and pharynx disappear. The micronucleus and contractile vacuole (?) persist. The macronucleus passes through a distinct metamorphosis. 15. Reorganization during permanent encystment in P. griseus is of the cytoplasmic or passive type. Nuclear reorganization or formation of a new micronucleus and a new macronucleus from the old micronucleus was not found to occur.