Aleut shows a remarkable alternation in its case and agreement patterns: roughly put, one pattern appears when a non—subject argument is syntactically unexpressed in a predicate, and the other pattern appears otherwise. This paper is devoted to an attempt to provide a coherent analysis for this alternation: the missing argument is analyzed as a pm which must move into a local relation with the highest T; in this position, it triggers additional agreement on the verb, and blocks normal case assignment to the subject (which then gets a different case). This movement is analogous to that of (potentially long) clitic movement, and its effects on the case and agreement patterns is shown to be similar to the wh-agreement pattern in Chamorro.