In the course of some observations on intestinal parasites of the frog, Rana clamitans, a flagellated organism was found in a considerable proportion of the freshly killed frogs. As I though this organism to be of some morphological interest, and as it was of common occurrence in the intestinal contents of different lots of frogs, a study of its morphology was undertaken. About thirty per cent of the frogs examined showed a mild infection with the flagellate in question. Fresh and stained preparations were made from the intestinal contents, but the organisms not being numerous, it was necessary to cultivate them in order to obtain material sufficiently rich for morphological study. Generous samples of the intestinal contents, which contained many Trichomonas and Hexamitus, as well as a few of the flagellates to be studied, were placed in different culture media. The flagellates grew in large numbers in a medium made of N.N.N. agar base slants, containing potassium phosphate as a buffer, to which was added about 3 cc. of physiologic salt solution containing 0.5 per cent of Difco, Loeffler's dried serum. Good growth was observed after four days incubation at room temperature. Numerous Trichomonas and Hexamitus were present in all the tubes, but no attempt was made to isolate the flagellate under study from these other organisms. The flagellate was studied in unstained fresh preparations, under a coverslip which was sealed to the slide with paraffin; and also by using vital stains, which lessened the activity of its movements and stained some of its structures. Thin films of weak alcoholic solutions or neutral