While most Mayan languages show an ergative-absolutive pattern of agreement in all main clauses, Chol shows what has been described as an aspect-based split: perfective clauses show an ergative-absolutive pattern while non-perfective clauses show what appears to be a nominative-accusative pattern. In this paper the author argues that Chol imperfective stems are formally possessed nominal arguments of a one-place predicate: the aspect marker mi. Under this analysis, all predicates in Chol show an ergative-absolutive pattern. The author argues further that the accusativity in Chol imperfective forms is an illusion, resulting from the fact that the "ERGATIVE" marker in imperfectives co-indexes a grammatical possessor, and that ERGATIVE = GENITIVE. Just as ergativity has been recently argued to arise in different ways (Aldridge 2004; Paul and Travis 2006; Legate 2008), a central claim of this paper is that the appearance of accusativity may also have more than one source. While the discussion focuses on Chol, it aims to provide insights into ergative and accusative systems more generally, and to make testable typological predictions.