Matses, a Panoan language spoken in Amazonian Peru and Brazil, has two fourth-person (= "interclausal third-person coreferential") pronouns, an ergative one (ambi) and an absolutive one (abi), that are used in subordinate clauses to indicate coreference with an argument in a higher clause. These fourth-person pronouns are typologically exceptional in that they do not also code possession or intraclausal coreference, do not play a role in indirect speech, and coreference is not restricted to matrix subjects. The interaction of argument structure and intra- and interclausal morphosyntax creates a complex but systematic set of rules governing the fourth person as opposed to the simple third person. The functions of the fourth person in Matses are explored in depth, particularly with respect to how they have motivated and shaped the evident diachronic development of the fourth-person forms from simple third-person pronouns that have been replaced by zero-anaphora. © 2008 by The University of Chicago. All rights reserved.