We report here on the behaviour of some Ciliates of te genus Balantidium in artificial culture media. For this study we have employed mainly Dobell and Laidlaw's amoeba medium to which rice starch was added. We investigated Balantidium coli, isolated from man; Balantidium sp., from the pig; and Balantidium simile, from Macaca mulatta, the Rhesus monkey. We could cultivate the human parasite very casily for a long time, transplanting it every 24 to 48 hours. Conjugation was very constantly observed in this species in culture about 24 hours old, either original cultures or transplants. The conjugants are found in pairs as small forms about 38,6 X 32,1 micra. The neutral individuals reach an average size of 75,5 X 57,8 micra. Small isolated preconjugant forms were also found in large numbers. These pre-conjugant forms in stained slides could be distinguished not only by their small size but also by the large size of their micronulcei which reached an average of 6 micra in diameter. The study of the stained material shows that the conjugants associate in pairs attaching themselves one to another by their peristomal region. Then their micronuclei divide twice by mitosis. From the four micronuclei thus three degenerate the remaining one dividing again in order to form the pronuclei around which a condensation of the cytoplasm is produced. After the exchange og the migratory pronuclei takes place, they are found in cach of the conjugants side by side with the stationary pronuclei. In each pair of pronuclei two mitotic spindles are formed, paralled one to another (gonomery) the fusion of both pronuclei (stationary and migratory) only then taking place. From both nuclei formed by this mitotic division simultaneous to the nuclear fusion, two new nuclei result one of which will become the new micronucleus. The other new nucleus divide again to form two placentae which will slowly grow and then fuse again to form the new macronucleus. The macronuclei originally found in both conjugants in spite of being found to degenerate by pycnosis are usually seen throughout almost all the conjugating process. Later on they break up in fragments which are reabsorved during the stage of pronuclear fusion. The species of Balantidium occurring in the pig was also very easily cultivated, conjugation being constantly observed in 24 hours old cultures. In Balantidium simile things were quite different. This species is difficult to cultivate, good results being obtained only when special technique is used as, for instance, when a buffer is added to the culture medium. In such a case abundant growth and multiplication of the Ciliate is seen after 24 hours either in the original cultures or in the transplants to the 4th. or 5th. subcultures. On the ground of a prolonged observation we can assert that Balantidium simile, in opposition to what one sees in the human and pig parasites, never conjugate in culture. In the original cultures and transplants of Balantidium simile the same phaenomena occur that we described from the faeces of Macaca mulatta, the Rhesus monkey, that is endomixis. On the stained slides we could observe the entire process, since the appearance of the initial forms (mitosis of the micronucleus and beginning of the degeneration of the macronucleus) until the final stages. Once more in this material we could see in a very clear way that the micronucleus is formed from one of the placentae, as we had the opportunity to show in a previous paper. Nelson in a recent paper (Observations and Experiments on Conjugation of the Balantidium from the Chimpanzee - American Journal of Hygiene, vol 20, n. 1, 1934), express his doubts about the phaenomena of endomixis we described in Balantidium simile. This author's opinion does not deserve discussion since he did not present the only proof that could invalidate our conclusions, that is the demonstration of the occurrence of pairs of conjugants, a fact he never succeeded to observe in the material at his disposal. As for our material that Nelson says to have examined it consists only in a few slides we gave to Professor Hegner when he visited Brazil. In these slides some stages of endomixis could be seen. On the ground of long lasting researches we conclude that Balantidium simile is a Ciliate in which conjugation seems to have disappeared and been relaced entirely and permanently by endomixis. According to our researches a similar fact accurs among another group of Ciliates, the families Cyathodiniidae and Enterophrydae. We propose to name azygotic or parthenogenetic the Ciliates in which this phaenomenon is observed.