Recent claims have been made,1 almost revolutionary in character from the standpoint of the science of medical protozoology, that protozoa heretofore regarded as living only in the large intestine also inhabit the biliary passages and gallbladder. This prompted us to seek to determine the types and number of protozoa living in the duodenum, and possibly in the gallbladder and biliary system, by the examination of duodenal aspirates from human adults, free from organic gastrointestinal and hepatobiliary diseases, presenting cyst or trophozoite forms of protozoa in discharged specimens of feces. As far as we can determine, this question— the comparative study of protozoa in duodenal aspirates in human subjects apparently free from organic disease but presenting protozoa in the feces—has never been investigated before. MATERIALS AND METHODS The fasting gastric residuum having been extracted, the tip of the duodenal bulb having been observed by means of the fluoroscope as being