These parasites were found in numbers in the evacuations of a man aged 30, who was born in Germany and had repeatedly travelled between Hamburg and North America, where he made long stays. The patient came to the Charité in Berlin to seek advice for constipation alternating with diarrhœa accompanied by abdominal pain. A second case (the parasite of which was described as _Colpoda cucullus_ by Schulz) was observed in a patient in the same institution. As, in both cases, the parasites only appeared during the diarrhœa, and disappeared as soon as the fæces had assumed a normal consistency, or could only be demonstrated in a few encysted specimens, it may be assumed that the small intestine or the duodenum is their habitat. Genus. *Nyctotherus*, Leidy, 1849. Flat, heterotrichous Infusoria, kidney- or bean-shaped. The peristome commences at the anterior pole of the body and extends along the concave side to the middle, where the oral aperture is situated. The cytopharynx is oblique and is more or less curved. The cytopyge is at the posterior extremity, where a single contractile vacuole is also situated. The macronucleus is almost in the centre of the parasite. The members of this genus live parasitically in the intestine of amphibia, insects and myriapods, and at least one species is found in man. *Nyctotherus faba*, Schaudinn, 1899. The body is bean-shaped, and a little flattened dorso-ventrally. It is 26 µ to 28 µ long, 16 µ to 18 µ broad, and 10 µ to 12 µ thick (fig. 116). The peristome is on the right border and extends to the middle; at the left there are large adoral cilia, the cilia on the right border not being larger than those on the body. The cytopharynx is short, slightly curved and turned backwards. The contractile vacuole is large, spherical, situated at the posterior extremity, and its contents are voided through the anus at its left. The macronucleus is in the centre of the body; it is globular (6 µ to 7 µ in size), and contains four or five chromatin masses. The micronucleus lies close to it, and is spherical or somewhat elongate measuring 1 µ to 1·5 µ (fig. 116). The cysts are oval. [Illustration: FIG. 116.--_Nyctotherus faba._ _P_, peristome; _N_, nucleus; _M_, micronucleus; _V_, contractile vacuole. (After Schaudinn.)] This species has hitherto only been seen once in the same patient in whom _Balantidium minutum_ was discovered. *Nyctotherus giganteus*, P. Krause, 1906.