Ergative case, the special case of transitive subjects, raises questions not only for the theory of case but also for theories of subjecthood and transitivity. This paper analyzes the case system of Nez Perce, a ”three-way ergative” language, with an eye towards a formalization of the category of transitive subject. I show that it is object agreement that is determinative of transitivity, and hence of ergative case, in Nez Perce. I further show that the transitivity condition on ergative case must be coupled with a criterion of subjecthood that makes reference to participation in subject agreement, not just to origin in a high argument-structural position. These two results suggest a formalization of the transitive subject as that argument uniquely accessing both high and low agreement information, the former through its (agreement-derived) connection with T and the latter through its origin in the specifier of a head associated with object agreement (v). In view of these findings, I argue that ergative case morphology should be analyzed not as the expression of a syntactic primitive but as the morphological spell-out of subject agreement and object agreement on a nominal.
