The Jivaro proper (Untsuri Suara) of eastern Ecuador have long been famous for their warlike practices, but the assumptions about reality upon which they predicate such behavior have not been systematically studied and analyzed. A very important series of these assumptions is to be found in the Jivaro ideas concerning souls. Among these concepts has recently been discovered a deep-seated belief that killing leads to the acquisition of souls which provide a supernatural power conferring immunity from death. The purpose of the present paper is to outline some of the basic elements of these beliefs with the hope of providing a greater insight into Jivaro behavior. The emphasis here will be on presenting new data collected in 1956-57, but the concluding paragraphs of the paper embody a commentary on relevant statements in the major publications dealing with this subject. At the outset it should be mentionedthat I took the two most useful workson the Jivaro (Karsten 1935; Stirling 1938) into the field and went over them in detail with native informants. Accordingly, points of differencebetween the following materialand that given by Karsten and Stirling should definitely be taken by the reader to indicate modification or refutation of the previously published statements, at least in regard to the Jivaro proper.