1. Cases of flagellate infection of man by Chilomastix mesnili have been reported from nearly every locality in the world. They are fairly abundant in the United States and are often confused with cases of infection by Trichomonas intestinalis. 2. The shape of Chilomastix is pyriform. The body contains the following structures: nucleus, centrosome, primary, secondary, and tertiary blepharoplasts, parabasal, parastyle, peristomal fiber, ventral cytostome, cytostomal flagellum, three anterior flagella, food vacuoles with bacterial inclusions, and a posterior spine-like process, the hold-fast organ. 3. The twisted shape in some flagellates in this case of infection is a temporary condition. It was seen in flagellates taken from the feces of the patient on only two occasions and was never observed among the cultured forms. 4. The cysts of Chilomastix are lemon-shaped and occurred irregularly in the stools of the patient. They have not been encountered thus far in the cultures. 5. Chilomastix mesnili was cultured continuously from January 27 to June 15, 1920, in an artificial medium composed of one part of human serum and four parts of Locke's solution with the addition of a small amount of dextrose. 6. Cultures sometimes remained viable for a period of 1 to 10 days, but generally for a period of 1 to 8 days. 7. The flagellates in the cultures increased in number during the 1st, 2nd, 3rd, and often the 4th days, after which there was a decrease until all had disappeared. 8. A period of lag, probably 13 hours in length, and followed by a period of great fecundity, characterized the growth of the flagellates during the 1st day in the cultures. 9. The number of flagellates in the cultures varied considerably and often reached 700,000 per cubic centimeter. 10. The bacteria served in part as the food of the flagellates. The bacterial organisms identified in the cultures were Bacillus coli communis, Bacillus coli communior, Bacterium aerogenes, Bacillus alkaligenes, and Bacillus proteus. The proteins of the blood serum, the mineral salts, and the dextrose in the medium may also have proved a source of food for the protozoa. 11. The products of bacterial metabolism, resulting from the growth of the bacteria, their fermentative activity, and their death may explain the rate of growth, reproduction, and death of the flagellates in the cultures. A certain amount of the products of decomposition together with the presence of the bacterial food supply may explain the rapid proliferation of the flagellates during the 1st day and the slower rate up to the 3rd or 4th day. An excess of these products may have inhibited the growth and multiplication of the protozoa, especially after the 3rd or 4th day, and ultimately may have brought about their death and disappearance from the cultures. 12. The initial alkalinity of the medium increased at the death of the culture. It is not believed that the change in alkalinity proved lethal to the protozoa. 13. Binary fission was often observed. The plane of division was median and longitudinal. A paradesmose was formed. 14. Multiple fission also occurred. The flagellate body, ameboid in movement, was a somatella comprised of four zooids. It contained four cytostomes, each with a cytostomal flagellum. The number of anterior flagella was not complete for each zooid. 15. Flagellates, joined in pairs, were often seen during the first 3 days in the cultures. They may represent conjugating forms, but as yet no evidence of the exchange of nuclear substance has been seen.