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Summary In this work, Trypanosoma ariarii, n. sp., has been described. This human parasite, found in the River Ariari Valley, Colombia, is characterized by an average size of 31 microns (in blood films), a nucleus anterior to the equator of the body, and a round, small, subterminal kinetoplast. T. ariarii shows division stages in the peripheral blood. Until now we have not found evolutionary forms in the viscera or tissues of experimental animals. Cebus fatuellus and the domestic dog have also been found naturally infected with T. ariarii. R. prolixus is experimentally infected quite easily with this protozoon. Also T. ariarii is cultivated in vitro without difficulty. So far, using parasites grown in cultures, we have only succeeded in infecting a few baby mice, two opossums (Didelphys paraguayensis paraguayensis), one rhesus monkey and human volunteers. During the period of observation (11 and 15 months) the experimental infection followed an asymptomatic course in the two volunteers in whom the infection was established. The frequency of human cases infected with T. ariarii in the Ariari Valley seems to be high. Out of 183 persons examined by blood cultures, 67 showed trypanosomes. It was possible to study completely the parasites isolated from 27 of these 67 infected people. The results were: 1 case with a mixed infection of both T. cruzi and T. ariarii, and 26 cases with single T. ariarii infections.

Alaria arisaemoides, n.sp., is described from the red fox, Vulpes fulva. The attachment of this parasite to its host and the host reaction are discussed in detail.